Include RE2 library

third_party/re2/re2/parse.cc

+// Copyright 2006 The RE2 Authors.  All Rights Reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Regular expression parser.
+
+// The parser is a simple precedence-based parser with a
+// manual stack.  The parsing work is done by the methods
+// of the ParseState class.  The Regexp::Parse function is
+// essentially just a lexer that calls the ParseState method
+// for each token.
+
+// The parser recognizes POSIX extended regular expressions
+// excluding backreferences, collating elements, and collating
+// classes.  It also allows the empty string as a regular expression
+// and recognizes the Perl escape sequences \d, \s, \w, \D, \S, and \W.
+// See regexp.h for rationale.
+
+#include <ctype.h>
+
+#include "util/util.h"
+#include "re2/regexp.h"
+#include "re2/stringpiece.h"
+#include "re2/unicode_casefold.h"
+#include "re2/unicode_groups.h"
+
+namespace re2 {
+
+// Regular expression parse state.
+// The list of parsed regexps so far is maintained as a vector of
+// Regexp pointers called the stack.  Left parenthesis and vertical
+// bar markers are also placed on the stack, as Regexps with
+// non-standard opcodes.
+// Scanning a left parenthesis causes the parser to push a left parenthesis
+// marker on the stack.
+// Scanning a vertical bar causes the parser to pop the stack until it finds a
+// vertical bar or left parenthesis marker (not popping the marker),
+// concatenate all the popped results, and push them back on
+// the stack (DoConcatenation).
+// Scanning a right parenthesis causes the parser to act as though it
+// has seen a vertical bar, which then leaves the top of the stack in the
+// form LeftParen regexp VerticalBar regexp VerticalBar ... regexp VerticalBar.
+// The parser pops all this off the stack and creates an alternation of the
+// regexps (DoAlternation).
+
+class Regexp::ParseState {
+ public:
+  ParseState(ParseFlags flags, const StringPiece& whole_regexp,
+             RegexpStatus* status);
+  ~ParseState();
+
+  ParseFlags flags() { return flags_; }
+  int rune_max() { return rune_max_; }
+
+  // Parse methods.  All public methods return a bool saying
+  // whether parsing should continue.  If a method returns
+  // false, it has set fields in *status_, and the parser
+  // should return NULL.
+
+  // Pushes the given regular expression onto the stack.
+  // Could check for too much memory used here.
+  bool PushRegexp(Regexp* re);
+
+  // Pushes the literal rune r onto the stack.
+  bool PushLiteral(Rune r);
+
+  // Pushes a regexp with the given op (and no args) onto the stack.
+  bool PushSimpleOp(RegexpOp op);
+
+  // Pushes a ^ onto the stack.
+  bool PushCarat();
+
+  // Pushes a \b (word == true) or \B (word == false) onto the stack.
+  bool PushWordBoundary(bool word);
+
+  // Pushes a $ onto the stack.
+  bool PushDollar();
+
+  // Pushes a . onto the stack
+  bool PushDot();
+
+  // Pushes a repeat operator regexp onto the stack.
+  // A valid argument for the operator must already be on the stack.
+  // s is the name of the operator, for use in error messages.
+  bool PushRepeatOp(RegexpOp op, const StringPiece& s, bool nongreedy);
+
+  // Pushes a repetition regexp onto the stack.
+  // A valid argument for the operator must already be on the stack.
+  bool PushRepetition(int min, int max, const StringPiece& s, bool nongreedy);
+
+  // Checks whether a particular regexp op is a marker.
+  bool IsMarker(RegexpOp op);
+
+  // Processes a left parenthesis in the input.
+  // Pushes a marker onto the stack.
+  bool DoLeftParen(const StringPiece& name);
+  bool DoLeftParenNoCapture();
+
+  // Processes a vertical bar in the input.
+  bool DoVerticalBar();
+
+  // Processes a right parenthesis in the input.
+  bool DoRightParen();
+
+  // Processes the end of input, returning the final regexp.
+  Regexp* DoFinish();
+
+  // Finishes the regexp if necessary, preparing it for use
+  // in a more complicated expression.
+  // If it is a CharClassBuilder, converts into a CharClass.
+  Regexp* FinishRegexp(Regexp*);
+
+  // These routines don't manipulate the parse stack
+  // directly, but they do need to look at flags_.
+  // ParseCharClass also manipulates the internals of Regexp
+  // while creating *out_re.
+
+  // Parse a character class into *out_re.
+  // Removes parsed text from s.
+  bool ParseCharClass(StringPiece* s, Regexp** out_re,
+                      RegexpStatus* status);
+
+  // Parse a character class character into *rp.
+  // Removes parsed text from s.
+  bool ParseCCCharacter(StringPiece* s, Rune *rp,
+                        const StringPiece& whole_class,
+                        RegexpStatus* status);
+
+  // Parse a character class range into rr.
+  // Removes parsed text from s.
+  bool ParseCCRange(StringPiece* s, RuneRange* rr,
+                    const StringPiece& whole_class,
+                    RegexpStatus* status);
+
+  // Parse a Perl flag set or non-capturing group from s.
+  bool ParsePerlFlags(StringPiece* s);
+
+
+  // Finishes the current concatenation,
+  // collapsing it into a single regexp on the stack.
+  void DoConcatenation();
+
+  // Finishes the current alternation,
+  // collapsing it to a single regexp on the stack.
+  void DoAlternation();
+
+  // Generalized DoAlternation/DoConcatenation.
+  void DoCollapse(RegexpOp op);
+
+  // Maybe concatenate Literals into LiteralString.
+  bool MaybeConcatString(int r, ParseFlags flags);
+
+private:
+  ParseFlags flags_;
+  StringPiece whole_regexp_;
+  RegexpStatus* status_;
+  Regexp* stacktop_;
+  int ncap_;  // number of capturing parens seen
+  int rune_max_;  // maximum char value for this encoding
+
+  DISALLOW_EVIL_CONSTRUCTORS(ParseState);
+};
+
+// Pseudo-operators - only on parse stack.
+const RegexpOp kLeftParen = static_cast<RegexpOp>(kMaxRegexpOp+1);
+const RegexpOp kVerticalBar = static_cast<RegexpOp>(kMaxRegexpOp+2);
+
+Regexp::ParseState::ParseState(ParseFlags flags,
+                               const StringPiece& whole_regexp,
+                               RegexpStatus* status)
+  : flags_(flags), whole_regexp_(whole_regexp),
+    status_(status), stacktop_(NULL), ncap_(0) {
+  if (flags_ & Latin1)
+    rune_max_ = 0xFF;
+  else
+    rune_max_ = Runemax;
+}
+
+// Cleans up by freeing all the regexps on the stack.
+Regexp::ParseState::~ParseState() {
+  Regexp* next;
+  for (Regexp* re = stacktop_; re != NULL; re = next) {
+    next = re->down_;
+    re->down_ = NULL;
+    if (re->op() == kLeftParen)
+      delete re->name_;
+    re->Decref();
+  }
+}
+
+// Finishes the regexp if necessary, preparing it for use in
+// a more complex expression.
+// If it is a CharClassBuilder, converts into a CharClass.
+Regexp* Regexp::ParseState::FinishRegexp(Regexp* re) {
+  if (re == NULL)
+    return NULL;
+  re->down_ = NULL;
+
+  if (re->op_ == kRegexpCharClass && re->ccb_ != NULL) {
+    CharClassBuilder* ccb = re->ccb_;
+    re->ccb_ = NULL;
+    re->cc_ = ccb->GetCharClass();
+    delete ccb;
+  }
+
+  return re;
+}
+
+// Pushes the given regular expression onto the stack.
+// Could check for too much memory used here.
+bool Regexp::ParseState::PushRegexp(Regexp* re) {
+  MaybeConcatString(-1, NoParseFlags);
+
+  // Special case: a character class of one character is just
+  // a literal.  This is a common idiom for escaping
+  // single characters (e.g., [.] instead of \.), and some
+  // analysis does better with fewer character classes.
+  // Similarly, [Aa] can be rewritten as a literal A with ASCII case folding.
+  if (re->op_ == kRegexpCharClass) {
+    if (re->ccb_->size() == 1) {
+      Rune r = re->ccb_->begin()->lo;
+      re->Decref();
+      re = new Regexp(kRegexpLiteral, flags_);
+      re->rune_ = r;
+    } else if (re->ccb_->size() == 2) {
+      Rune r = re->ccb_->begin()->lo;
+      if ('A' <= r && r <= 'Z' && re->ccb_->Contains(r + 'a' - 'A')) {
+        re->Decref();
+        re = new Regexp(kRegexpLiteral, flags_ | FoldCase);
+        re->rune_ = r + 'a' - 'A';
+      }
+    }
+  }
+
+  if (!IsMarker(re->op()))
+    re->simple_ = re->ComputeSimple();
+  re->down_ = stacktop_;
+  stacktop_ = re;
+  return true;
+}
+
+// Searches the case folding tables and returns the CaseFold* that contains r.
+// If there isn't one, returns the CaseFold* with smallest f->lo bigger than r.
+// If there isn't one, returns NULL.
+CaseFold* LookupCaseFold(CaseFold *f, int n, Rune r) {
+  CaseFold* ef = f + n;
+
+  // Binary search for entry containing r.
+  while (n > 0) {
+    int m = n/2;
+    if (f[m].lo <= r && r <= f[m].hi)
+      return &f[m];
+    if (r < f[m].lo) {
+      n = m;
+    } else {
+      f += m+1;
+      n -= m+1;
+    }
+  }
+
+  // There is no entry that contains r, but f points
+  // where it would have been.  Unless f points at
+  // the end of the array, it points at the next entry
+  // after r.
+  if (f < ef)
+    return f;
+
+  // No entry contains r; no entry contains runes > r.
+  return NULL;
+}
+
+// Returns the result of applying the fold f to the rune r.
+Rune ApplyFold(CaseFold *f, Rune r) {
+  switch (f->delta) {
+    default:
+      return r + f->delta;
+
+    case EvenOddSkip:  // even <-> odd but only applies to every other
+      if ((r - f->lo) % 2)
+        return r;
+      // fall through
+    case EvenOdd:  // even <-> odd
+      if (r%2 == 0)
+        return r + 1;
+      return r - 1;
+
+    case OddEvenSkip:  // odd <-> even but only applies to every other
+      if ((r - f->lo) % 2)
+        return r;
+      // fall through
+    case OddEven:  // odd <-> even
+      if (r%2 == 1)
+        return r + 1;
+      return r - 1;
+  }
+}
+
+// Returns the next Rune in r's folding cycle (see unicode_casefold.h).
+// Examples:
+//   CycleFoldRune('A') = 'a'
+//   CycleFoldRune('a') = 'A'
+//
+//   CycleFoldRune('K') = 'k'
+//   CycleFoldRune('k') = 0x212A (Kelvin)
+//   CycleFoldRune(0x212A) = 'K'
+//
+//   CycleFoldRune('?') = '?'
+Rune CycleFoldRune(Rune r) {
+  CaseFold* f = LookupCaseFold(unicode_casefold, num_unicode_casefold, r);
+  if (f == NULL || r < f->lo)
+    return r;
+  return ApplyFold(f, r);
+}
+
+// Add lo-hi to the class, along with their fold-equivalent characters.
+// If lo-hi is already in the class, assume that the fold-equivalent
+// chars are there too, so there's no work to do.
+static void AddFoldedRange(CharClassBuilder* cc, Rune lo, Rune hi, int depth) {
+  // AddFoldedRange calls itself recursively for each rune in the fold cycle.
+  // Most folding cycles are small: there aren't any bigger than four in the
+  // current Unicode tables.  make_unicode_casefold.py checks that
+  // the cycles are not too long, and we double-check here using depth.
+  if (depth > 10) {
+    LOG(DFATAL) << "AddFoldedRange recurses too much.";
+    return;
+  }
+
+  if (!cc->AddRange(lo, hi))  // lo-hi was already there? we're done
+    return;
+
+  while (lo <= hi) {
+    CaseFold* f = LookupCaseFold(unicode_casefold, num_unicode_casefold, lo);
+    if (f == NULL)  // lo has no fold, nor does anything above lo
+      break;
+    if (lo < f->lo) {  // lo has no fold; next rune with a fold is f->lo
+      lo = f->lo;
+      continue;
+    }
+
+    // Add in the result of folding the range lo - f->hi
+    // and that range's fold, recursively.
+    Rune lo1 = lo;
+    Rune hi1 = min<Rune>(hi, f->hi);
+    switch (f->delta) {
+      default:
+        lo1 += f->delta;
+        hi1 += f->delta;
+        break;
+      case EvenOdd:
+        if (lo1%2 == 1)
+          lo1--;
+        if (hi1%2 == 0)
+          hi1++;
+        break;
+      case OddEven:
+        if (lo1%2 == 0)
+          lo1--;
+        if (hi1%2 == 1)
+          hi1++;
+        break;
+    }
+    AddFoldedRange(cc, lo1, hi1, depth+1);
+
+    // Pick up where this fold left off.
+    lo = f->hi + 1;
+  }
+}
+
+// Pushes the literal rune r onto the stack.
+bool Regexp::ParseState::PushLiteral(Rune r) {
+  // Do case folding if needed.
+  if ((flags_ & FoldCase) && CycleFoldRune(r) != r) {
+    Regexp* re = new Regexp(kRegexpCharClass, flags_ & ~FoldCase);
+    re->ccb_ = new CharClassBuilder;
+    Rune r1 = r;
+    do {
+      if (!(flags_ & NeverNL) || r != '\n') {
+        re->ccb_->AddRange(r, r);
+      }
+      r = CycleFoldRune(r);
+    } while (r != r1);
+    re->ccb_->RemoveAbove(rune_max_);
+    return PushRegexp(re);
+  }
+
+  // Exclude newline if applicable.
+  if ((flags_ & NeverNL) && r == '\n')
+    return PushRegexp(new Regexp(kRegexpNoMatch, flags_));
+
+  // No fancy stuff worked.  Ordinary literal.
+  if (MaybeConcatString(r, flags_))
+    return true;
+
+  Regexp* re = new Regexp(kRegexpLiteral, flags_);
+  re->rune_ = r;
+  return PushRegexp(re);
+}
+
+// Pushes a ^ onto the stack.
+bool Regexp::ParseState::PushCarat() {
+  if (flags_ & OneLine) {
+    return PushSimpleOp(kRegexpBeginText);
+  }
+  return PushSimpleOp(kRegexpBeginLine);
+}
+
+// Pushes a \b or \B onto the stack.
+bool Regexp::ParseState::PushWordBoundary(bool word) {
+  if (word)
+    return PushSimpleOp(kRegexpWordBoundary);
+  return PushSimpleOp(kRegexpNoWordBoundary);
+}
+
+// Pushes a $ onto the stack.
+bool Regexp::ParseState::PushDollar() {
+  if (flags_ & OneLine) {
+    // Clumsy marker so that MimicsPCRE() can tell whether
+    // this kRegexpEndText was a $ and not a \z.
+    Regexp::ParseFlags oflags = flags_;
+    flags_ = flags_ | WasDollar;
+    bool ret = PushSimpleOp(kRegexpEndText);
+    flags_ = oflags;
+    return ret;
+  }
+  return PushSimpleOp(kRegexpEndLine);
+}
+
+// Pushes a . onto the stack.
+bool Regexp::ParseState::PushDot() {
+  if ((flags_ & DotNL) && !(flags_ & NeverNL))
+    return PushSimpleOp(kRegexpAnyChar);
+  // Rewrite . into [^\n]
+  Regexp* re = new Regexp(kRegexpCharClass, flags_ & ~FoldCase);
+  re->ccb_ = new CharClassBuilder;
+  re->ccb_->AddRange(0, '\n' - 1);
+  re->ccb_->AddRange('\n' + 1, rune_max_);
+  return PushRegexp(re);
+}
+
+// Pushes a regexp with the given op (and no args) onto the stack.
+bool Regexp::ParseState::PushSimpleOp(RegexpOp op) {
+  Regexp* re = new Regexp(op, flags_);
+  return PushRegexp(re);
+}
+
+// Pushes a repeat operator regexp onto the stack.
+// A valid argument for the operator must already be on the stack.
+// The char c is the name of the operator, for use in error messages.
+bool Regexp::ParseState::PushRepeatOp(RegexpOp op, const StringPiece& s,
+                                      bool nongreedy) {
+  if (stacktop_ == NULL || IsMarker(stacktop_->op())) {
+    status_->set_code(kRegexpRepeatArgument);
+    status_->set_error_arg(s);
+    return false;
+  }
+  Regexp::ParseFlags fl = flags_;
+  if (nongreedy)
+    fl = fl ^ NonGreedy;
+  Regexp* re = new Regexp(op, fl);
+  re->AllocSub(1);
+  re->down_ = stacktop_->down_;
+  re->sub()[0] = FinishRegexp(stacktop_);
+  re->simple_ = re->ComputeSimple();
+  stacktop_ = re;
+  return true;
+}
+
+// Pushes a repetition regexp onto the stack.
+// A valid argument for the operator must already be on the stack.
+bool Regexp::ParseState::PushRepetition(int min, int max,
+                                        const StringPiece& s,
+                                        bool nongreedy) {
+  if ((max != -1 && max < min) || min > 1000 || max > 1000) {
+    status_->set_code(kRegexpRepeatSize);
+    status_->set_error_arg(s);
+    return false;
+  }
+  if (stacktop_ == NULL || IsMarker(stacktop_->op())) {
+    status_->set_code(kRegexpRepeatArgument);
+    status_->set_error_arg(s);
+    return false;
+  }
+  Regexp::ParseFlags fl = flags_;
+  if (nongreedy)
+    fl = fl ^ NonGreedy;
+  Regexp* re = new Regexp(kRegexpRepeat, fl);
+  re->min_ = min;
+  re->max_ = max;
+  re->AllocSub(1);
+  re->down_ = stacktop_->down_;
+  re->sub()[0] = FinishRegexp(stacktop_);
+  re->simple_ = re->ComputeSimple();
+
+  stacktop_ = re;
+  return true;
+}
+
+// Checks whether a particular regexp op is a marker.
+bool Regexp::ParseState::IsMarker(RegexpOp op) {
+  return op >= kLeftParen;
+}
+
+// Processes a left parenthesis in the input.
+// Pushes a marker onto the stack.
+bool Regexp::ParseState::DoLeftParen(const StringPiece& name) {
+  Regexp* re = new Regexp(kLeftParen, flags_);
+  re->cap_ = ++ncap_;
+  if (name.data() != NULL)
+    re->name_ = new string(name.as_string());
+  return PushRegexp(re);
+}
+
+// Pushes a non-capturing marker onto the stack.
+bool Regexp::ParseState::DoLeftParenNoCapture() {
+  Regexp* re = new Regexp(kLeftParen, flags_);
+  re->cap_ = -1;
+  return PushRegexp(re);
+}
+
+// Adds r to cc, along with r's upper case if foldascii is set.
+static void AddLiteral(CharClassBuilder* cc, Rune r, bool foldascii) {
+  cc->AddRange(r, r);
+  if (foldascii && 'a' <= r && r <= 'z')
+    cc->AddRange(r + 'A' - 'a', r + 'A' - 'a');
+}
+
+// Processes a vertical bar in the input.
+bool Regexp::ParseState::DoVerticalBar() {
+  MaybeConcatString(-1, NoParseFlags);
+  DoConcatenation();
+
+  // Below the vertical bar is a list to alternate.
+  // Above the vertical bar is a list to concatenate.
+  // We just did the concatenation, so either swap
+  // the result below the vertical bar or push a new
+  // vertical bar on the stack.
+  Regexp* r1;
+  Regexp* r2;
+  if ((r1 = stacktop_) != NULL &&
+      (r2 = stacktop_->down_) != NULL &&
+      r2->op() == kVerticalBar) {
+    // If above and below vertical bar are literal or char class,
+    // can merge into a single char class.
+    Regexp* r3;
+    if ((r1->op() == kRegexpLiteral ||
+         r1->op() == kRegexpCharClass ||
+         r1->op() == kRegexpAnyChar) &&
+        (r3 = r2->down_) != NULL) {
+      Rune rune;
+      switch (r3->op()) {
+        case kRegexpLiteral:  // convert to char class
+          rune = r3->rune_;
+          r3->op_ = kRegexpCharClass;
+          r3->cc_ = NULL;
+          r3->ccb_ = new CharClassBuilder;
+          AddLiteral(r3->ccb_, rune, r3->parse_flags_ & Regexp::FoldCase);
+          // fall through
+        case kRegexpCharClass:
+          if (r1->op() == kRegexpLiteral)
+            AddLiteral(r3->ccb_, r1->rune_,
+                       r1->parse_flags_ & Regexp::FoldCase);
+          else if (r1->op() == kRegexpCharClass)
+            r3->ccb_->AddCharClass(r1->ccb_);
+          if (r1->op() == kRegexpAnyChar || r3->ccb_->full()) {
+            delete r3->ccb_;
+            r3->ccb_ = NULL;
+            r3->op_ = kRegexpAnyChar;
+          }
+          // fall through
+        case kRegexpAnyChar:
+          // pop r1
+          stacktop_ = r2;
+          r1->Decref();
+          return true;
+        default:
+          break;
+      }
+    }
+
+    // Swap r1 below vertical bar (r2).
+    r1->down_ = r2->down_;
+    r2->down_ = r1;
+    stacktop_ = r2;
+    return true;
+  }
+  return PushSimpleOp(kVerticalBar);
+}
+
+// Processes a right parenthesis in the input.
+bool Regexp::ParseState::DoRightParen() {
+  // Finish the current concatenation and alternation.
+  DoAlternation();
+
+  // The stack should be: LeftParen regexp
+  // Remove the LeftParen, leaving the regexp,
+  // parenthesized.
+  Regexp* r1;
+  Regexp* r2;
+  if ((r1 = stacktop_) == NULL ||
+      (r2 = r1->down_) == NULL ||
+      r2->op() != kLeftParen) {
+    status_->set_code(kRegexpMissingParen);
+    status_->set_error_arg(whole_regexp_);
+    return false;
+  }
+
+  // Pop off r1, r2.  Will Decref or reuse below.
+  stacktop_ = r2->down_;
+
+  // Restore flags from when paren opened.
+  Regexp* re = r2;
+  flags_ = re->parse_flags();
+
+  // Rewrite LeftParen as capture if needed.
+  if (re->cap_ > 0) {
+    re->op_ = kRegexpCapture;
+    // re->cap_ is already set
+    re->AllocSub(1);
+    re->sub()[0] = FinishRegexp(r1);
+    re->simple_ = re->ComputeSimple();
+  } else {
+    re->Decref();
+    re = r1;
+  }
+  return PushRegexp(re);
+}
+
+// Processes the end of input, returning the final regexp.
+Regexp* Regexp::ParseState::DoFinish() {
+  DoAlternation();
+  Regexp* re = stacktop_;
+  if (re != NULL && re->down_ != NULL) {
+    status_->set_code(kRegexpMissingParen);
+    status_->set_error_arg(whole_regexp_);
+    return NULL;
+  }
+  stacktop_ = NULL;
+  return FinishRegexp(re);
+}
+
+// Returns the leading regexp that re starts with.
+// The returned Regexp* points into a piece of re,
+// so it must not be used after the caller calls re->Decref().
+Regexp* Regexp::LeadingRegexp(Regexp* re) {
+  if (re->op() == kRegexpEmptyMatch)
+    return NULL;
+  if (re->op() == kRegexpConcat && re->nsub() >= 2) {
+    Regexp** sub = re->sub();
+    if (sub[0]->op() == kRegexpEmptyMatch)
+      return NULL;
+    return sub[0];
+  }
+  return re;
+}
+
+// Removes LeadingRegexp(re) from re and returns what's left.
+// Consumes the reference to re and may edit it in place.
+// If caller wants to hold on to LeadingRegexp(re),
+// must have already Incref'ed it.
+Regexp* Regexp::RemoveLeadingRegexp(Regexp* re) {
+  if (re->op() == kRegexpEmptyMatch)
+    return re;
+  if (re->op() == kRegexpConcat && re->nsub() >= 2) {
+    Regexp** sub = re->sub();
+    if (sub[0]->op() == kRegexpEmptyMatch)
+      return re;
+    sub[0]->Decref();
+    sub[0] = NULL;
+    if (re->nsub() == 2) {
+      // Collapse concatenation to single regexp.
+      Regexp* nre = sub[1];
+      sub[1] = NULL;
+      re->Decref();
+      return nre;
+    }
+    // 3 or more -> 2 or more.
+    re->nsub_--;
+    memmove(sub, sub + 1, re->nsub_ * sizeof sub[0]);
+    return re;
+  }
+  Regexp::ParseFlags pf = re->parse_flags();
+  re->Decref();
+  return new Regexp(kRegexpEmptyMatch, pf);
+}
+
+// Returns the leading string that re starts with.
+// The returned Rune* points into a piece of re,
+// so it must not be used after the caller calls re->Decref().
+Rune* Regexp::LeadingString(Regexp* re, int *nrune,
+                            Regexp::ParseFlags *flags) {
+  while (re->op() == kRegexpConcat && re->nsub() > 0)
+    re = re->sub()[0];
+
+  *flags = static_cast<Regexp::ParseFlags>(re->parse_flags_ & Regexp::FoldCase);
+
+  if (re->op() == kRegexpLiteral) {
+    *nrune = 1;
+    return &re->rune_;
+  }
+
+  if (re->op() == kRegexpLiteralString) {
+    *nrune = re->nrunes_;
+    return re->runes_;
+  }
+
+  *nrune = 0;
+  return NULL;
+}
+
+// Removes the first n leading runes from the beginning of re.
+// Edits re in place.
+void Regexp::RemoveLeadingString(Regexp* re, int n) {
+  // Chase down concats to find first string.
+  // For regexps generated by parser, nested concats are
+  // flattened except when doing so would overflow the 16-bit
+  // limit on the size of a concatenation, so we should never
+  // see more than two here.
+  Regexp* stk[4];
+  int d = 0;
+  while (re->op() == kRegexpConcat) {
+    if (d < arraysize(stk))
+      stk[d++] = re;
+    re = re->sub()[0];
+  }
+
+  // Remove leading string from re.
+  if (re->op() == kRegexpLiteral) {
+    re->rune_ = 0;
+    re->op_ = kRegexpEmptyMatch;
+  } else if (re->op() == kRegexpLiteralString) {
+    if (n >= re->nrunes_) {
+      delete[] re->runes_;
+      re->runes_ = NULL;
+      re->nrunes_ = 0;
+      re->op_ = kRegexpEmptyMatch;
+    } else if (n == re->nrunes_ - 1) {
+      Rune rune = re->runes_[re->nrunes_ - 1];
+      delete[] re->runes_;
+      re->runes_ = NULL;
+      re->nrunes_ = 0;
+      re->rune_ = rune;
+      re->op_ = kRegexpLiteral;
+    } else {
+      re->nrunes_ -= n;
+      memmove(re->runes_, re->runes_ + n, re->nrunes_ * sizeof re->runes_[0]);
+    }
+  }
+
+  // If re is now empty, concatenations might simplify too.
+  while (d-- > 0) {
+    re = stk[d];
+    Regexp** sub = re->sub();
+    if (sub[0]->op() == kRegexpEmptyMatch) {
+      sub[0]->Decref();
+      sub[0] = NULL;
+      // Delete first element of concat.
+      switch (re->nsub()) {
+        case 0:
+        case 1:
+          // Impossible.
+          LOG(DFATAL) << "Concat of " << re->nsub();
+          re->submany_ = NULL;
+          re->op_ = kRegexpEmptyMatch;
+          break;
+
+        case 2: {
+          // Replace re with sub[1].
+          Regexp* old = sub[1];
+          sub[1] = NULL;
+          re->Swap(old);
+          old->Decref();
+          break;
+        }
+
+        default:
+          // Slide down.
+          re->nsub_--;
+          memmove(sub, sub + 1, re->nsub_ * sizeof sub[0]);
+          break;
+      }
+    }
+  }
+}
+
+// Factors common prefixes from alternation.
+// For example,
+//     ABC|ABD|AEF|BCX|BCY
+// simplifies to
+//     A(B(C|D)|EF)|BC(X|Y)
+// which the normal parse state routines will further simplify to
+//     A(B[CD]|EF)|BC[XY]
+//
+// Rewrites sub to contain simplified list to alternate and returns
+// the new length of sub.  Adjusts reference counts accordingly
+// (incoming sub[i] decremented, outgoing sub[i] incremented).
+
+// It's too much of a pain to write this code with an explicit stack,
+// so instead we let the caller specify a maximum depth and
+// don't simplify beyond that.  There are around 15 words of local
+// variables and parameters in the frame, so allowing 8 levels
+// on a 64-bit machine is still less than a kilobyte of stack and
+// probably enough benefit for practical uses.
+const int kFactorAlternationMaxDepth = 8;
+
+int Regexp::FactorAlternation(
+    Regexp** sub, int n,
+    Regexp::ParseFlags altflags) {
+  return FactorAlternationRecursive(sub, n, altflags,
+                                    kFactorAlternationMaxDepth);
+}
+
+int Regexp::FactorAlternationRecursive(
+    Regexp** sub, int n,
+    Regexp::ParseFlags altflags,
+    int maxdepth) {
+
+  if (maxdepth <= 0)
+    return n;
+
+  // Round 1: Factor out common literal prefixes.
+  Rune *rune = NULL;
+  int nrune = 0;
+  Regexp::ParseFlags runeflags = Regexp::NoParseFlags;
+  int start = 0;
+  int out = 0;
+  for (int i = 0; i <= n; i++) {
+    // Invariant: what was in sub[0:start] has been Decref'ed
+    // and that space has been reused for sub[0:out] (out <= start).
+    //
+    // Invariant: sub[start:i] consists of regexps that all begin
+    // with the string rune[0:nrune].
+
+    Rune* rune_i = NULL;
+    int nrune_i = 0;
+    Regexp::ParseFlags runeflags_i = Regexp::NoParseFlags;
+    if (i < n) {
+      rune_i = LeadingString(sub[i], &nrune_i, &runeflags_i);
+      if (runeflags_i == runeflags) {
+        int same = 0;
+        while (same < nrune && same < nrune_i && rune[same] == rune_i[same])
+          same++;
+        if (same > 0) {
+          // Matches at least one rune in current range.  Keep going around.
+          nrune = same;
+          continue;
+        }
+      }
+    }
+
+    // Found end of a run with common leading literal string:
+    // sub[start:i] all begin with rune[0:nrune] but sub[i]
+    // does not even begin with rune[0].
+    //
+    // Factor out common string and append factored expression to sub[0:out].
+    if (i == start) {
+      // Nothing to do - first iteration.
+    } else if (i == start+1) {
+      // Just one: don't bother factoring.
+      sub[out++] = sub[start];
+    } else {
+      // Construct factored form: prefix(suffix1|suffix2|...)
+      Regexp* x[2];  // x[0] = prefix, x[1] = suffix1|suffix2|...
+      x[0] = LiteralString(rune, nrune, runeflags);
+      for (int j = start; j < i; j++)
+        RemoveLeadingString(sub[j], nrune);
+      int nn = FactorAlternationRecursive(sub + start, i - start, altflags,
+                                          maxdepth - 1);
+      x[1] = AlternateNoFactor(sub + start, nn, altflags);
+      sub[out++] = Concat(x, 2, altflags);
+    }
+
+    // Prepare for next round (if there is one).
+    if (i < n) {
+      start = i;
+      rune = rune_i;
+      nrune = nrune_i;
+      runeflags = runeflags_i;
+    }
+  }
+  n = out;
+
+  // Round 2: Factor out common complex prefixes,
+  // just the first piece of each concatenation,
+  // whatever it is.  This is good enough a lot of the time.
+  start = 0;
+  out = 0;
+  Regexp* first = NULL;
+  for (int i = 0; i <= n; i++) {
+    // Invariant: what was in sub[0:start] has been Decref'ed
+    // and that space has been reused for sub[0:out] (out <= start).
+    //
+    // Invariant: sub[start:i] consists of regexps that all begin with first.
+
+    Regexp* first_i = NULL;
+    if (i < n) {
+      first_i = LeadingRegexp(sub[i]);
+      if (first != NULL && Regexp::Equal(first, first_i)) {
+        continue;
+      }
+    }
+
+    // Found end of a run with common leading regexp:
+    // sub[start:i] all begin with first but sub[i] does not.
+    //
+    // Factor out common regexp and append factored expression to sub[0:out].
+    if (i == start) {
+      // Nothing to do - first iteration.
+    } else if (i == start+1) {
+      // Just one: don't bother factoring.
+      sub[out++] = sub[start];
+    } else {
+      // Construct factored form: prefix(suffix1|suffix2|...)
+      Regexp* x[2];  // x[0] = prefix, x[1] = suffix1|suffix2|...
+      x[0] = first->Incref();
+      for (int j = start; j < i; j++)
+        sub[j] = RemoveLeadingRegexp(sub[j]);
+      int nn = FactorAlternationRecursive(sub + start, i - start, altflags,
+                                   maxdepth - 1);
+      x[1] = AlternateNoFactor(sub + start, nn, altflags);
+      sub[out++] = Concat(x, 2, altflags);
+    }
+
+    // Prepare for next round (if there is one).
+    if (i < n) {
+      start = i;
+      first = first_i;
+    }
+  }
+  n = out;
+
+  // Round 3: Collapse runs of single literals into character classes.
+  start = 0;
+  out = 0;
+  for (int i = 0; i <= n; i++) {
+    // Invariant: what was in sub[0:start] has been Decref'ed
+    // and that space has been reused for sub[0:out] (out <= start).
+    //
+    // Invariant: sub[start:i] consists of regexps that are either
+    // literal runes or character classes.
+
+    if (i < n &&
+        (sub[i]->op() == kRegexpLiteral ||
+         sub[i]->op() == kRegexpCharClass))
+      continue;
+
+    // sub[i] is not a char or char class;
+    // emit char class for sub[start:i]...
+    if (i == start) {
+      // Nothing to do.
+    } else if (i == start+1) {
+      sub[out++] = sub[start];
+    } else {
+      // Make new char class.
+      CharClassBuilder ccb;
+      for (int j = start; j < i; j++) {
+        Regexp* re = sub[j];
+        if (re->op() == kRegexpCharClass) {
+          CharClass* cc = re->cc();
+          for (CharClass::iterator it = cc->begin(); it != cc->end(); ++it)
+            ccb.AddRange(it->lo, it->hi);
+        } else if (re->op() == kRegexpLiteral) {
+          ccb.AddRangeFlags(re->rune(), re->rune(), re->parse_flags());
+        } else {
+          LOG(DFATAL) << "RE2: unexpected op: " << re->op() << " "
+                      << re->ToString();
+        }
+        re->Decref();
+      }
+      sub[out++] = NewCharClass(ccb.GetCharClass(), altflags);
+    }
+
+    // ... and then emit sub[i].
+    if (i < n)
+      sub[out++] = sub[i];
+    start = i+1;
+  }
+  n = out;
+
+  // Round 4: Collapse runs of empty matches into single empty match.
+  start = 0;
+  out = 0;
+  for (int i = 0; i < n; i++) {
+    if (i + 1 < n &&
+        sub[i]->op() == kRegexpEmptyMatch &&
+        sub[i+1]->op() == kRegexpEmptyMatch) {
+      sub[i]->Decref();
+      continue;
+    }
+    sub[out++] = sub[i];
+  }
+  n = out;
+
+  return n;
+}
+
+// Collapse the regexps on top of the stack, down to the
+// first marker, into a new op node (op == kRegexpAlternate
+// or op == kRegexpConcat).
+void Regexp::ParseState::DoCollapse(RegexpOp op) {
+  // Scan backward to marker, counting children of composite.
+  int n = 0;
+  Regexp* next = NULL;
+  Regexp* sub;
+  for (sub = stacktop_; sub != NULL && !IsMarker(sub->op()); sub = next) {
+    next = sub->down_;
+    if (sub->op_ == op)
+      n += sub->nsub_;
+    else
+      n++;
+  }
+
+  // If there's just one child, leave it alone.
+  // (Concat of one thing is that one thing; alternate of one thing is same.)
+  if (stacktop_ != NULL && stacktop_->down_ == next)
+    return;
+
+  // Construct op (alternation or concatenation), flattening op of op.
+  Regexp** subs = new Regexp*[n];
+  next = NULL;
+  int i = n;
+  for (sub = stacktop_; sub != NULL && !IsMarker(sub->op()); sub = next) {
+    next = sub->down_;
+    if (sub->op_ == op) {
+      Regexp** sub_subs = sub->sub();
+      for (int k = sub->nsub_ - 1; k >= 0; k--)
+        subs[--i] = sub_subs[k]->Incref();
+      sub->Decref();
+    } else {
+      subs[--i] = FinishRegexp(sub);
+    }
+  }
+
+  Regexp* re = ConcatOrAlternate(op, subs, n, flags_, true);
+  delete[] subs;
+  re->simple_ = re->ComputeSimple();
+  re->down_ = next;
+  stacktop_ = re;
+}
+
+// Finishes the current concatenation,
+// collapsing it into a single regexp on the stack.
+void Regexp::ParseState::DoConcatenation() {
+  Regexp* r1 = stacktop_;
+  if (r1 == NULL || IsMarker(r1->op())) {
+    // empty concatenation is special case
+    Regexp* re = new Regexp(kRegexpEmptyMatch, flags_);
+    PushRegexp(re);
+  }
+  DoCollapse(kRegexpConcat);
+}
+
+// Finishes the current alternation,
+// collapsing it to a single regexp on the stack.
+void Regexp::ParseState::DoAlternation() {
+  DoVerticalBar();
+  // Now stack top is kVerticalBar.
+  Regexp* r1 = stacktop_;
+  stacktop_ = r1->down_;
+  r1->Decref();
+  DoCollapse(kRegexpAlternate);
+}
+
+// Incremental conversion of concatenated literals into strings.
+// If top two elements on stack are both literal or string,
+// collapse into single string.
+// Don't walk down the stack -- the parser calls this frequently
+// enough that below the bottom two is known to be collapsed.
+// Only called when another regexp is about to be pushed
+// on the stack, so that the topmost literal is not being considered.
+// (Otherwise ab* would turn into (ab)*.)
+// If r >= 0, consider pushing a literal r on the stack.
+// Return whether that happened.
+bool Regexp::ParseState::MaybeConcatString(int r, ParseFlags flags) {
+  Regexp* re1;
+  Regexp* re2;
+  if ((re1 = stacktop_) == NULL || (re2 = re1->down_) == NULL)
+    return false;
+
+  if (re1->op_ != kRegexpLiteral && re1->op_ != kRegexpLiteralString)
+    return false;
+  if (re2->op_ != kRegexpLiteral && re2->op_ != kRegexpLiteralString)
+    return false;
+  if ((re1->parse_flags_ & FoldCase) != (re2->parse_flags_ & FoldCase))
+    return false;
+
+  if (re2->op_ == kRegexpLiteral) {
+    // convert into string
+    Rune rune = re2->rune_;
+    re2->op_ = kRegexpLiteralString;
+    re2->nrunes_ = 0;
+    re2->runes_ = NULL;
+    re2->AddRuneToString(rune);
+  }
+
+  // push re1 into re2.
+  if (re1->op_ == kRegexpLiteral) {
+    re2->AddRuneToString(re1->rune_);
+  } else {
+    for (int i = 0; i < re1->nrunes_; i++)
+      re2->AddRuneToString(re1->runes_[i]);
+    re1->nrunes_ = 0;
+    delete[] re1->runes_;
+    re1->runes_ = NULL;
+  }
+
+  // reuse re1 if possible
+  if (r >= 0) {
+    re1->op_ = kRegexpLiteral;
+    re1->rune_ = r;
+    re1->parse_flags_ = flags;
+    return true;
+  }
+
+  stacktop_ = re2;
+  re1->Decref();
+  return false;
+}
+
+// Lexing routines.
+
+// Parses a decimal integer, storing it in *n.
+// Sets *s to span the remainder of the string.
+// Sets *out_re to the regexp for the class.
+static bool ParseInteger(StringPiece* s, int* np) {
+  if (s->size() == 0 || !isdigit((*s)[0] & 0xFF))
+    return false;
+  // Disallow leading zeros.
+  if (s->size() >= 2 && (*s)[0] == '0' && isdigit((*s)[1] & 0xFF))
+    return false;
+  int n = 0;
+  int c;
+  while (s->size() > 0 && isdigit(c = (*s)[0] & 0xFF)) {
+    // Avoid overflow.
+    if (n >= 100000000)
+      return false;
+    n = n*10 + c - '0';
+    s->remove_prefix(1);  // digit
+  }
+  *np = n;
+  return true;
+}
+
+// Parses a repetition suffix like {1,2} or {2} or {2,}.
+// Sets *s to span the remainder of the string on success.
+// Sets *lo and *hi to the given range.
+// In the case of {2,}, the high number is unbounded;
+// sets *hi to -1 to signify this.
+// {,2} is NOT a valid suffix.
+// The Maybe in the name signifies that the regexp parse
+// doesn't fail even if ParseRepetition does, so the StringPiece
+// s must NOT be edited unless MaybeParseRepetition returns true.
+static bool MaybeParseRepetition(StringPiece* sp, int* lo, int* hi) {
+  StringPiece s = *sp;
+  if (s.size() == 0 || s[0] != '{')
+    return false;
+  s.remove_prefix(1);  // '{'
+  if (!ParseInteger(&s, lo))
+    return false;
+  if (s.size() == 0)
+    return false;
+  if (s[0] == ',') {
+    s.remove_prefix(1);  // ','
+    if (s.size() == 0)
+      return false;
+    if (s[0] == '}') {
+      // {2,} means at least 2
+      *hi = -1;
+    } else {
+      // {2,4} means 2, 3, or 4.
+      if (!ParseInteger(&s, hi))
+        return false;
+    }
+  } else {
+    // {2} means exactly two
+    *hi = *lo;
+  }
+  if (s.size() == 0 || s[0] != '}')
+    return false;
+  s.remove_prefix(1);  // '}'
+  *sp = s;
+  return true;
+}
+
+// Removes the next Rune from the StringPiece and stores it in *r.
+// Returns number of bytes removed from sp.
+// Behaves as though there is a terminating NUL at the end of sp.
+// Argument order is backwards from usual Google style
+// but consistent with chartorune.
+static int StringPieceToRune(Rune *r, StringPiece *sp, RegexpStatus* status) {
+  int n;
+  if (fullrune(sp->data(), sp->size())) {
+    n = chartorune(r, sp->data());
+    if (!(n == 1 && *r == Runeerror)) {  // no decoding error
+      sp->remove_prefix(n);
+      return n;
+    }
+  }
+
+  status->set_code(kRegexpBadUTF8);
+  status->set_error_arg(NULL);
+  return -1;
+}
+
+// Return whether name is valid UTF-8.
+// If not, set status to kRegexpBadUTF8.
+static bool IsValidUTF8(const StringPiece& s, RegexpStatus* status) {
+  StringPiece t = s;
+  Rune r;
+  while (t.size() > 0) {
+    if (StringPieceToRune(&r, &t, status) < 0)
+      return false;
+  }
+  return true;
+}
+
+// Is c a hex digit?
+static int IsHex(int c) {
+  return ('0' <= c && c <= '9') ||
+         ('A' <= c && c <= 'F') ||
+         ('a' <= c && c <= 'f');
+}
+
+// Convert hex digit to value.
+static int UnHex(int c) {
+  if ('0' <= c && c <= '9')
+    return c - '0';
+  if ('A' <= c && c <= 'F')
+    return c - 'A' + 10;
+  if ('a' <= c && c <= 'f')
+    return c - 'a' + 10;
+  LOG(DFATAL) << "Bad hex digit " << c;
+  return 0;
+}
+
+// Parse an escape sequence (e.g., \n, \{).
+// Sets *s to span the remainder of the string.
+// Sets *rp to the named character.
+static bool ParseEscape(StringPiece* s, Rune* rp,
+                        RegexpStatus* status, int rune_max) {
+  const char* begin = s->begin();
+  if (s->size() < 1 || (*s)[0] != '\\') {
+    // Should not happen - caller always checks.
+    status->set_code(kRegexpInternalError);
+    status->set_error_arg(NULL);
+    return false;
+  }
+  if (s->size() < 2) {
+    status->set_code(kRegexpTrailingBackslash);
+    status->set_error_arg(NULL);
+    return false;
+  }
+  Rune c, c1;
+  s->remove_prefix(1);  // backslash
+  if (StringPieceToRune(&c, s, status) < 0)
+    return false;
+  int code;
+  switch (c) {
+    default:
+      if (c < Runeself && !isalpha(c) && !isdigit(c)) {
+        // Escaped non-word characters are always themselves.
+        // PCRE is not quite so rigorous: it accepts things like
+        // \q, but we don't.  We once rejected \_, but too many
+        // programs and people insist on using it, so allow \_.
+        *rp = c;
+        return true;
+      }
+      goto BadEscape;
+
+    // Octal escapes.
+    case '1':
+    case '2':
+    case '3':
+    case '4':
+    case '5':
+    case '6':
+    case '7':
+      // Single non-zero octal digit is a backreference; not supported.
+      if (s->size() == 0 || (*s)[0] < '0' || (*s)[0] > '7')
+        goto BadEscape;
+      // fall through
+    case '0':
+      // consume up to three octal digits; already have one.
+      code = c - '0';
+      if (s->size() > 0 && '0' <= (c = (*s)[0]) && c <= '7') {
+        code = code * 8 + c - '0';
+        s->remove_prefix(1);  // digit
+        if (s->size() > 0) {
+          c = (*s)[0];
+          if ('0' <= c && c <= '7') {
+            code = code * 8 + c - '0';
+            s->remove_prefix(1);  // digit
+          }
+        }
+      }
+      *rp = code;
+      return true;
+
+    // Hexadecimal escapes
+    case 'x':
+      if (s->size() == 0)
+        goto BadEscape;
+      if (StringPieceToRune(&c, s, status) < 0)
+        return false;
+      if (c == '{') {
+        // Any number of digits in braces.
+        // Update n as we consume the string, so that
+        // the whole thing gets shown in the error message.
+        // Perl accepts any text at all; it ignores all text
+        // after the first non-hex digit.  We require only hex digits,
+        // and at least one.
+        if (StringPieceToRune(&c, s, status) < 0)
+          return false;
+        int nhex = 0;
+        code = 0;
+        while (IsHex(c)) {
+          nhex++;
+          code = code * 16 + UnHex(c);
+          if (code > rune_max)
+            goto BadEscape;
+          if (s->size() == 0)
+            goto BadEscape;
+          if (StringPieceToRune(&c, s, status) < 0)
+            return false;
+        }
+        if (c != '}' || nhex == 0)
+          goto BadEscape;
+        *rp = code;
+        return true;
+      }
+      // Easy case: two hex digits.
+      if (s->size() == 0)
+        goto BadEscape;
+      if (StringPieceToRune(&c1, s, status) < 0)
+        return false;
+      if (!IsHex(c) || !IsHex(c1))
+        goto BadEscape;
+      *rp = UnHex(c) * 16 + UnHex(c1);
+      return true;
+
+    // C escapes.
+    case 'n':
+      *rp = '\n';
+      return true;
+    case 'r':
+      *rp = '\r';
+      return true;
+    case 't':
+      *rp = '\t';
+      return true;
+
+    // Less common C escapes.
+    case 'a':
+      *rp = '\a';
+      return true;
+    case 'f':
+      *rp = '\f';
+      return true;
+    case 'v':
+      *rp = '\v';
+      return true;
+
+    // This code is disabled to avoid misparsing
+    // the Perl word-boundary \b as a backspace
+    // when in POSIX regexp mode.  Surprisingly,
+    // in Perl, \b means word-boundary but [\b]
+    // means backspace.  We don't support that:
+    // if you want a backspace embed a literal
+    // backspace character or use \x08.
+    //
+    // case 'b':
+    //   *rp = '\b';
+    //   return true;
+  }
+
+  LOG(DFATAL) << "Not reached in ParseEscape.";
+
+BadEscape:
+  // Unrecognized escape sequence.
+  status->set_code(kRegexpBadEscape);
+  status->set_error_arg(StringPiece(begin, s->data() - begin));
+  return false;
+}
+
+// Add a range to the character class, but exclude newline if asked.
+// Also handle case folding.
+void CharClassBuilder::AddRangeFlags(
+    Rune lo, Rune hi, Regexp::ParseFlags parse_flags) {
+
+  // Take out \n if the flags say so.
+  bool cutnl = !(parse_flags & Regexp::ClassNL) ||
+               (parse_flags & Regexp::NeverNL);
+  if (cutnl && lo <= '\n' && '\n' <= hi) {
+    if (lo < '\n')
+      AddRangeFlags(lo, '\n' - 1, parse_flags);
+    if (hi > '\n')
+      AddRangeFlags('\n' + 1, hi, parse_flags);
+    return;
+  }
+
+  // If folding case, add fold-equivalent characters too.
+  if (parse_flags & Regexp::FoldCase)
+    AddFoldedRange(this, lo, hi, 0);
+  else
+    AddRange(lo, hi);
+}
+
+// Look for a group with the given name.
+static UGroup* LookupGroup(const StringPiece& name,
+                           UGroup *groups, int ngroups) {
+  // Simple name lookup.
+  for (int i = 0; i < ngroups; i++)
+    if (StringPiece(groups[i].name) == name)
+      return &groups[i];
+  return NULL;
+}
+
+// Fake UGroup containing all Runes
+static URange16 any16[] = { { 0, 65535 } };
+static URange32 any32[] = { { 65536, Runemax } };
+static UGroup anygroup = { "Any", +1, any16, 1, any32, 1 };
+
+// Look for a POSIX group with the given name (e.g., "[:^alpha:]")
+static UGroup* LookupPosixGroup(const StringPiece& name) {
+  return LookupGroup(name, posix_groups, num_posix_groups);
+}
+
+static UGroup* LookupPerlGroup(const StringPiece& name) {
+  return LookupGroup(name, perl_groups, num_perl_groups);
+}
+
+// Look for a Unicode group with the given name (e.g., "Han")
+static UGroup* LookupUnicodeGroup(const StringPiece& name) {
+  // Special case: "Any" means any.
+  if (name == StringPiece("Any"))
+    return &anygroup;
+  return LookupGroup(name, unicode_groups, num_unicode_groups);
+}
+
+// Add a UGroup or its negation to the character class.
+static void AddUGroup(CharClassBuilder *cc, UGroup *g, int sign,
+                      Regexp::ParseFlags parse_flags) {
+  if (sign == +1) {
+    for (int i = 0; i < g->nr16; i++) {
+      cc->AddRangeFlags(g->r16[i].lo, g->r16[i].hi, parse_flags);
+    }
+    for (int i = 0; i < g->nr32; i++) {
+      cc->AddRangeFlags(g->r32[i].lo, g->r32[i].hi, parse_flags);
+    }
+  } else {
+    if (parse_flags & Regexp::FoldCase) {
+      // Normally adding a case-folded group means
+      // adding all the extra fold-equivalent runes too.
+      // But if we're adding the negation of the group,
+      // we have to exclude all the runes that are fold-equivalent
+      // to what's already missing.  Too hard, so do in two steps.
+      CharClassBuilder ccb1;
+      AddUGroup(&ccb1, g, +1, parse_flags);
+      ccb1.Negate();
+      cc->AddCharClass(&ccb1);
+      return;
+    }
+    int next = 0;
+    for (int i = 0; i < g->nr16; i++) {
+      if (next < g->r16[i].lo)
+        cc->AddRangeFlags(next, g->r16[i].lo - 1, parse_flags);
+      next = g->r16[i].hi + 1;
+    }
+    for (int i = 0; i < g->nr32; i++) {
+      if (next < g->r32[i].lo)
+        cc->AddRangeFlags(next, g->r32[i].lo - 1, parse_flags);
+      next = g->r32[i].hi + 1;
+    }
+    if (next <= Runemax)
+      cc->AddRangeFlags(next, Runemax, parse_flags);
+  }
+}
+
+// Maybe parse a Perl character class escape sequence.
+// Only recognizes the Perl character classes (\d \s \w \D \S \W),
+// not the Perl empty-string classes (\b \B \A \Z \z).
+// On success, sets *s to span the remainder of the string
+// and returns the corresponding UGroup.
+// The StringPiece must *NOT* be edited unless the call succeeds.
+UGroup* MaybeParsePerlCCEscape(StringPiece* s, Regexp::ParseFlags parse_flags) {
+  if (!(parse_flags & Regexp::PerlClasses))
+    return NULL;
+  if (s->size() < 2 || (*s)[0] != '\\')
+    return NULL;
+  // Could use StringPieceToRune, but there aren't
+  // any non-ASCII Perl group names.
+  StringPiece name(s->begin(), 2);
+  UGroup *g = LookupPerlGroup(name);
+  if (g == NULL)
+    return NULL;
+  s->remove_prefix(name.size());
+  return g;
+}
+
+enum ParseStatus {
+  kParseOk,  // Did some parsing.
+  kParseError,  // Found an error.
+  kParseNothing,  // Decided not to parse.
+};
+
+// Maybe parses a Unicode character group like \p{Han} or \P{Han}
+// (the latter is a negated group).
+ParseStatus ParseUnicodeGroup(StringPiece* s, Regexp::ParseFlags parse_flags,
+                              CharClassBuilder *cc,
+                              RegexpStatus* status) {
+  // Decide whether to parse.
+  if (!(parse_flags & Regexp::UnicodeGroups))
+    return kParseNothing;
+  if (s->size() < 2 || (*s)[0] != '\\')
+    return kParseNothing;
+  Rune c = (*s)[1];
+  if (c != 'p' && c != 'P')
+    return kParseNothing;
+
+  // Committed to parse.  Results:
+  int sign = +1;  // -1 = negated char class
+  if (c == 'P')
+    sign = -1;
+  StringPiece seq = *s;  // \p{Han} or \pL
+  StringPiece name;  // Han or L
+  s->remove_prefix(2);  // '\\', 'p'
+
+  if (!StringPieceToRune(&c, s, status))
+    return kParseError;
+  if (c != '{') {
+    // Name is the bit of string we just skipped over for c.
+    const char* p = seq.begin() + 2;
+    name = StringPiece(p, s->begin() - p);
+  } else {
+    // Name is in braces. Look for closing }
+    int end = s->find('}', 0);
+    if (end == s->npos) {
+      if (!IsValidUTF8(seq, status))
+        return kParseError;
+      status->set_code(kRegexpBadCharRange);
+      status->set_error_arg(seq);
+      return kParseError;
+    }
+    name = StringPiece(s->begin(), end);  // without '}'
+    s->remove_prefix(end + 1);  // with '}'
+    if (!IsValidUTF8(name, status))
+      return kParseError;
+  }
+
+  // Chop seq where s now begins.
+  seq = StringPiece(seq.begin(), s->begin() - seq.begin());
+
+  // Look up group
+  if (name.size() > 0 && name[0] == '^') {
+    sign = -sign;
+    name.remove_prefix(1);  // '^'
+  }
+  UGroup *g = LookupUnicodeGroup(name);
+  if (g == NULL) {
+    status->set_code(kRegexpBadCharRange);
+    status->set_error_arg(seq);
+    return kParseError;
+  }
+
+  AddUGroup(cc, g, sign, parse_flags);
+  return kParseOk;
+}
+
+// Parses a character class name like [:alnum:].
+// Sets *s to span the remainder of the string.
+// Adds the ranges corresponding to the class to ranges.
+static ParseStatus ParseCCName(StringPiece* s, Regexp::ParseFlags parse_flags,
+                               CharClassBuilder *cc,
+                               RegexpStatus* status) {
+  // Check begins with [:
+  const char* p = s->data();
+  const char* ep = s->data() + s->size();
+  if (ep - p < 2 || p[0] != '[' || p[1] != ':')
+    return kParseNothing;
+
+  // Look for closing :].
+  const char* q;
+  for (q = p+2; q <= ep-2 && (*q != ':' || *(q+1) != ']'); q++)
+    ;
+
+  // If no closing :], then ignore.
+  if (q > ep-2)
+    return kParseNothing;
+
+  // Got it.  Check that it's valid.
+  q += 2;
+  StringPiece name(p, q-p);
+
+  UGroup *g = LookupPosixGroup(name);
+  if (g == NULL) {
+    status->set_code(kRegexpBadCharRange);
+    status->set_error_arg(name);
+    return kParseError;
+  }
+
+  s->remove_prefix(name.size());
+  AddUGroup(cc, g, g->sign, parse_flags);
+  return kParseOk;
+}
+
+// Parses a character inside a character class.
+// There are fewer special characters here than in the rest of the regexp.
+// Sets *s to span the remainder of the string.
+// Sets *rp to the character.
+bool Regexp::ParseState::ParseCCCharacter(StringPiece* s, Rune *rp,
+                                          const StringPiece& whole_class,
+                                          RegexpStatus* status) {
+  if (s->size() == 0) {
+    status->set_code(kRegexpMissingBracket);
+    status->set_error_arg(whole_class);
+    return false;
+  }
+
+  // Allow regular escape sequences even though
+  // many need not be escaped in this context.
+  if (s->size() >= 1 && (*s)[0] == '\\')
+    return ParseEscape(s, rp, status, rune_max_);
+
+  // Otherwise take the next rune.
+  return StringPieceToRune(rp, s, status) >= 0;
+}
+
+// Parses a character class character, or, if the character
+// is followed by a hyphen, parses a character class range.
+// For single characters, rr->lo == rr->hi.
+// Sets *s to span the remainder of the string.
+// Sets *rp to the character.
+bool Regexp::ParseState::ParseCCRange(StringPiece* s, RuneRange* rr,
+                                      const StringPiece& whole_class,
+                                      RegexpStatus* status) {
+  StringPiece os = *s;
+  if (!ParseCCCharacter(s, &rr->lo, whole_class, status))
+    return false;
+  // [a-] means (a|-), so check for final ].
+  if (s->size() >= 2 && (*s)[0] == '-' && (*s)[1] != ']') {
+    s->remove_prefix(1);  // '-'
+    if (!ParseCCCharacter(s, &rr->hi, whole_class, status))
+      return false;
+    if (rr->hi < rr->lo) {
+      status->set_code(kRegexpBadCharRange);
+      status->set_error_arg(StringPiece(os.data(), s->data() - os.data()));
+      return false;
+    }
+  } else {
+    rr->hi = rr->lo;
+  }
+  return true;
+}
+
+// Parses a possibly-negated character class expression like [^abx-z[:digit:]].
+// Sets *s to span the remainder of the string.
+// Sets *out_re to the regexp for the class.
+bool Regexp::ParseState::ParseCharClass(StringPiece* s,
+                                        Regexp** out_re,
+                                        RegexpStatus* status) {
+  StringPiece whole_class = *s;
+  if (s->size() == 0 || (*s)[0] != '[') {
+    // Caller checked this.
+    status->set_code(kRegexpInternalError);
+    status->set_error_arg(NULL);
+    return false;
+  }
+  bool negated = false;
+  Regexp* re = new Regexp(kRegexpCharClass, flags_ & ~FoldCase);
+  re->ccb_ = new CharClassBuilder;
+  s->remove_prefix(1);  // '['
+  if (s->size() > 0 && (*s)[0] == '^') {
+    s->remove_prefix(1);  // '^'
+    negated = true;
+    if (!(flags_ & ClassNL) || (flags_ & NeverNL)) {
+      // If NL can't match implicitly, then pretend
+      // negated classes include a leading \n.
+      re->ccb_->AddRange('\n', '\n');
+    }
+  }
+  bool first = true;  // ] is okay as first char in class
+  while (s->size() > 0 && ((*s)[0] != ']' || first)) {
+    // - is only okay unescaped as first or last in class.
+    // Except that Perl allows - anywhere.
+    if ((*s)[0] == '-' && !first && !(flags_&PerlX) &&
+        (s->size() == 1 || (*s)[1] != ']')) {
+      StringPiece t = *s;
+      t.remove_prefix(1);  // '-'
+      Rune r;
+      int n = StringPieceToRune(&r, &t, status);
+      if (n < 0) {
+        re->Decref();
+        return false;
+      }
+      status->set_code(kRegexpBadCharRange);
+      status->set_error_arg(StringPiece(s->data(), 1+n));
+      re->Decref();
+      return false;
+    }
+    first = false;
+
+    // Look for [:alnum:] etc.
+    if (s->size() > 2 && (*s)[0] == '[' && (*s)[1] == ':') {
+      switch (ParseCCName(s, flags_, re->ccb_, status)) {
+        case kParseOk:
+          continue;
+        case kParseError:
+          re->Decref();
+          return false;
+        case kParseNothing:
+          break;
+      }
+    }
+
+    // Look for Unicode character group like \p{Han}
+    if (s->size() > 2 &&
+        (*s)[0] == '\\' &&
+        ((*s)[1] == 'p' || (*s)[1] == 'P')) {
+      switch (ParseUnicodeGroup(s, flags_, re->ccb_, status)) {
+        case kParseOk:
+          continue;
+        case kParseError:
+          re->Decref();
+          return false;
+        case kParseNothing:
+          break;
+      }
+    }
+
+    // Look for Perl character class symbols (extension).
+    UGroup *g = MaybeParsePerlCCEscape(s, flags_);
+    if (g != NULL) {
+      AddUGroup(re->ccb_, g, g->sign, flags_);
+      continue;
+    }
+
+    // Otherwise assume single character or simple range.
+    RuneRange rr;
+    if (!ParseCCRange(s, &rr, whole_class, status)) {
+      re->Decref();
+      return false;
+    }
+    // AddRangeFlags is usually called in response to a class like
+    // \p{Foo} or [[:foo:]]; for those, it filters \n out unless
+    // Regexp::ClassNL is set.  In an explicit range or singleton
+    // like we just parsed, we do not filter \n out, so set ClassNL
+    // in the flags.
+    re->ccb_->AddRangeFlags(rr.lo, rr.hi, flags_ | Regexp::ClassNL);
+  }
+  if (s->size() == 0) {
+    status->set_code(kRegexpMissingBracket);
+    status->set_error_arg(whole_class);
+    re->Decref();
+    return false;
+  }
+  s->remove_prefix(1);  // ']'
+
+  if (negated)
+    re->ccb_->Negate();
+  re->ccb_->RemoveAbove(rune_max_);
+
+  *out_re = re;
+  return true;
+}
+
+// Is this a valid capture name?  [A-Za-z0-9_]+
+// PCRE limits names to 32 bytes.
+// Python rejects names starting with digits.
+// We don't enforce either of those.
+static bool IsValidCaptureName(const StringPiece& name) {
+  if (name.size() == 0)
+    return false;
+  for (int i = 0; i < name.size(); i++) {
+    int c = name[i];
+    if (('0' <= c && c <= '9') ||
+        ('a' <= c && c <= 'z') ||
+        ('A' <= c && c <= 'Z') ||
+        c == '_')
+      continue;
+    return false;
+  }
+  return true;
+}
+
+// Parses a Perl flag setting or non-capturing group or both,
+// like (?i) or (?: or (?i:.  Removes from s, updates parse state.
+// The caller must check that s begins with "(?".
+// Returns true on success.  If the Perl flag is not
+// well-formed or not supported, sets status_ and returns false.
+bool Regexp::ParseState::ParsePerlFlags(StringPiece* s) {
+  StringPiece t = *s;
+
+  // Caller is supposed to check this.
+  if (!(flags_ & PerlX) || t.size() < 2 || t[0] != '(' || t[1] != '?') {
+    LOG(DFATAL) << "Bad call to ParseState::ParsePerlFlags";
+    status_->set_code(kRegexpInternalError);
+    return false;
+  }
+
+  t.remove_prefix(2);  // "(?"
+
+  // Check for named captures, first introduced in Python's regexp library.
+  // As usual, there are three slightly different syntaxes:
+  //
+  //   (?P<name>expr)   the original, introduced by Python
+  //   (?<name>expr)    the .NET alteration, adopted by Perl 5.10
+  //   (?'name'expr)    another .NET alteration, adopted by Perl 5.10
+  //
+  // Perl 5.10 gave in and implemented the Python version too,
+  // but they claim that the last two are the preferred forms.
+  // PCRE and languages based on it (specifically, PHP and Ruby)
+  // support all three as well.  EcmaScript 4 uses only the Python form.
+  //
+  // In both the open source world (via Code Search) and the
+  // Google source tree, (?P<expr>name) is the dominant form,
+  // so that's the one we implement.  One is enough.
+  if (t.size() > 2 && t[0] == 'P' && t[1] == '<') {
+    // Pull out name.
+    int end = t.find('>', 2);
+    if (end == t.npos) {
+      if (!IsValidUTF8(*s, status_))
+        return false;
+      status_->set_code(kRegexpBadNamedCapture);
+      status_->set_error_arg(*s);
+      return false;
+    }
+
+    // t is "P<name>...", t[end] == '>'
+    StringPiece capture(t.begin()-2, end+3);  // "(?P<name>"
+    StringPiece name(t.begin()+2, end-2);     // "name"
+    if (!IsValidUTF8(name, status_))
+      return false;
+    if (!IsValidCaptureName(name)) {
+      status_->set_code(kRegexpBadNamedCapture);
+      status_->set_error_arg(capture);
+      return false;
+    }
+
+    if (!DoLeftParen(name)) {
+      // DoLeftParen's failure set status_.
+      return false;
+    }
+
+    s->remove_prefix(capture.end() - s->begin());
+    return true;
+  }
+
+  bool negated = false;
+  bool sawflags = false;
+  int nflags = flags_;
+  Rune c;
+  for (bool done = false; !done; ) {
+    if (t.size() == 0)
+      goto BadPerlOp;
+    if (StringPieceToRune(&c, &t, status_) < 0)
+      return false;
+    switch (c) {
+      default:
+        goto BadPerlOp;
+
+      // Parse flags.
+      case 'i':
+        sawflags = true;
+        if (negated)
+          nflags &= ~FoldCase;
+        else
+          nflags |= FoldCase;
+        break;
+
+      case 'm':  // opposite of our OneLine
+        sawflags = true;
+        if (negated)
+          nflags |= OneLine;
+        else
+          nflags &= ~OneLine;
+        break;
+
+      case 's':
+        sawflags = true;
+        if (negated)
+          nflags &= ~DotNL;
+        else
+          nflags |= DotNL;
+        break;
+
+      case 'U':
+        sawflags = true;
+        if (negated)
+          nflags &= ~NonGreedy;
+        else
+          nflags |= NonGreedy;
+        break;
+
+      // Negation
+      case '-':
+        if (negated)
+          goto BadPerlOp;
+        negated = true;
+        sawflags = false;
+        break;
+
+      // Open new group.
+      case ':':
+        if (!DoLeftParenNoCapture()) {
+          // DoLeftParenNoCapture's failure set status_.
+          return false;
+        }
+        done = true;
+        break;
+
+      // Finish flags.
+      case ')':
+        done = true;
+        break;
+    }
+  }
+
+  if (negated && !sawflags)
+    goto BadPerlOp;
+
+  flags_ = static_cast<Regexp::ParseFlags>(nflags);
+  *s = t;
+  return true;
+
+BadPerlOp:
+  status_->set_code(kRegexpBadPerlOp);
+  status_->set_error_arg(StringPiece(s->begin(), t.begin() - s->begin()));
+  return false;
+}
+
+// Converts latin1 (assumed to be encoded as Latin1 bytes)
+// into UTF8 encoding in string.
+// Can't use EncodingUtils::EncodeLatin1AsUTF8 because it is
+// deprecated and because it rejects code points 0x80-0x9F.
+void ConvertLatin1ToUTF8(const StringPiece& latin1, string* utf) {
+  char buf[UTFmax];
+
+  utf->clear();
+  for (int i = 0; i < latin1.size(); i++) {
+    Rune r = latin1[i] & 0xFF;
+    int n = runetochar(buf, &r);
+    utf->append(buf, n);
+  }
+}
+
+// Parses the regular expression given by s,
+// returning the corresponding Regexp tree.
+// The caller must Decref the return value when done with it.
+// Returns NULL on error.
+Regexp* Regexp::Parse(const StringPiece& s, ParseFlags global_flags,
+                      RegexpStatus* status) {
+  // Make status non-NULL (easier on everyone else).
+  RegexpStatus xstatus;
+  if (status == NULL)
+    status = &xstatus;
+
+  ParseState ps(global_flags, s, status);
+  StringPiece t = s;
+
+  // Convert regexp to UTF-8 (easier on the rest of the parser).
+  if (global_flags & Latin1) {
+    string* tmp = new string;
+    ConvertLatin1ToUTF8(t, tmp);
+    status->set_tmp(tmp);
+    t = *tmp;
+  }
+
+  if (global_flags & Literal) {
+    // Special parse loop for literal string.
+    while (t.size() > 0) {
+      Rune r;
+      if (StringPieceToRune(&r, &t, status) < 0)
+        return NULL;
+      if (!ps.PushLiteral(r))
+        return NULL;
+    }
+    return ps.DoFinish();
+  }
+
+  StringPiece lastunary = NULL;
+  while (t.size() > 0) {
+    StringPiece isunary = NULL;
+    switch (t[0]) {
+      default: {
+        Rune r;
+        if (StringPieceToRune(&r, &t, status) < 0)
+          return NULL;
+        if (!ps.PushLiteral(r))
+          return NULL;
+        break;
+      }
+
+      case '(':
+        // "(?" introduces Perl escape.
+        if ((ps.flags() & PerlX) && (t.size() >= 2 && t[1] == '?')) {
+          // Flag changes and non-capturing groups.
+          if (!ps.ParsePerlFlags(&t))
+            return NULL;
+          break;
+        }
+        if (!ps.DoLeftParen(NULL))
+          return NULL;
+        t.remove_prefix(1);  // '('
+        break;
+
+      case '|':
+        if (!ps.DoVerticalBar())
+          return NULL;
+        t.remove_prefix(1);  // '|'
+        break;
+
+      case ')':
+        if (!ps.DoRightParen())
+          return NULL;
+        t.remove_prefix(1);  // ')'
+        break;
+
+      case '^':  // Beginning of line.
+        if (!ps.PushCarat())
+          return NULL;
+        t.remove_prefix(1);  // '^'
+        break;
+
+      case '$':  // End of line.
+        if (!ps.PushDollar())
+          return NULL;
+        t.remove_prefix(1);  // '$'
+        break;
+
+      case '.':  // Any character (possibly except newline).
+        if (!ps.PushDot())
+          return NULL;
+        t.remove_prefix(1);  // '.'
+        break;
+
+      case '[': {  // Character class.
+        Regexp* re;
+        if (!ps.ParseCharClass(&t, &re, status))
+          return NULL;
+        if (!ps.PushRegexp(re))
+          return NULL;
+        break;
+      }
+
+      case '*': {  // Zero or more.
+        RegexpOp op;
+        op = kRegexpStar;
+        goto Rep;
+      case '+':  // One or more.
+        op = kRegexpPlus;
+        goto Rep;
+      case '?':  // Zero or one.
+        op = kRegexpQuest;
+        goto Rep;
+      Rep:
+        StringPiece opstr = t;
+        bool nongreedy = false;
+        t.remove_prefix(1);  // '*' or '+' or '?'
+        if (ps.flags() & PerlX) {
+          if (t.size() > 0 && t[0] == '?') {
+            nongreedy = true;
+            t.remove_prefix(1);  // '?'
+          }
+          if (lastunary.size() > 0) {
+            // In Perl it is not allowed to stack repetition operators:
+            //   a** is a syntax error, not a double-star.
+            // (and a++ means something else entirely, which we don't support!)
+            status->set_code(kRegexpRepeatOp);
+            status->set_error_arg(StringPiece(lastunary.begin(),
+                                              t.begin() - lastunary.begin()));
+            return NULL;
+          }
+        }
+        opstr.set(opstr.data(), t.data() - opstr.data());
+        if (!ps.PushRepeatOp(op, opstr, nongreedy))
+          return NULL;
+        isunary = opstr;
+        break;
+      }
+
+      case '{': {  // Counted repetition.
+        int lo, hi;
+        StringPiece opstr = t;
+        if (!MaybeParseRepetition(&t, &lo, &hi)) {
+          // Treat like a literal.
+          if (!ps.PushLiteral('{'))
+            return NULL;
+          t.remove_prefix(1);  // '{'
+          break;
+        }
+        bool nongreedy = false;
+        if (ps.flags() & PerlX) {
+          if (t.size() > 0 && t[0] == '?') {
+            nongreedy = true;
+            t.remove_prefix(1);  // '?'
+          }
+          if (lastunary.size() > 0) {
+            // Not allowed to stack repetition operators.
+            status->set_code(kRegexpRepeatOp);
+            status->set_error_arg(StringPiece(lastunary.begin(),
+                                              t.begin() - lastunary.begin()));
+            return NULL;
+          }
+        }
+        opstr.set(opstr.data(), t.data() - opstr.data());
+        if (!ps.PushRepetition(lo, hi, opstr, nongreedy))
+          return NULL;
+        isunary = opstr;
+        break;
+      }
+
+      case '\\': {  // Escaped character or Perl sequence.
+        // \b and \B: word boundary or not
+        if ((ps.flags() & Regexp::PerlB) &&
+            t.size() >= 2 && (t[1] == 'b' || t[1] == 'B')) {
+          if (!ps.PushWordBoundary(t[1] == 'b'))
+            return NULL;
+          t.remove_prefix(2);  // '\\', 'b'
+          break;
+        }
+
+        if ((ps.flags() & Regexp::PerlX) && t.size() >= 2) {
+          if (t[1] == 'A') {
+            if (!ps.PushSimpleOp(kRegexpBeginText))
+              return NULL;
+            t.remove_prefix(2);  // '\\', 'A'
+            break;
+          }
+          if (t[1] == 'z') {
+            if (!ps.PushSimpleOp(kRegexpEndText))
+              return NULL;
+            t.remove_prefix(2);  // '\\', 'z'
+            break;
+          }
+          // Do not recognize \Z, because this library can't
+          // implement the exact Perl/PCRE semantics.
+          // (This library treats "(?-m)$" as \z, even though
+          // in Perl and PCRE it is equivalent to \Z.)
+
+          if (t[1] == 'C') {  // \C: any byte [sic]
+            if (!ps.PushSimpleOp(kRegexpAnyByte))
+              return NULL;
+            t.remove_prefix(2);  // '\\', 'C'
+            break;
+          }
+
+          if (t[1] == 'Q') {  // \Q ... \E: the ... is always literals
+            t.remove_prefix(2);  // '\\', 'Q'
+            while (t.size() > 0) {
+              if (t.size() >= 2 && t[0] == '\\' && t[1] == 'E') {
+                t.remove_prefix(2);  // '\\', 'E'
+                break;
+              }
+              Rune r;
+              if (StringPieceToRune(&r, &t, status) < 0)
+                return NULL;
+              if (!ps.PushLiteral(r))
+                return NULL;
+            }
+            break;
+          }
+        }
+
+        if (t.size() >= 2 && (t[1] == 'p' || t[1] == 'P')) {
+          Regexp* re = new Regexp(kRegexpCharClass, ps.flags() & ~FoldCase);
+          re->ccb_ = new CharClassBuilder;
+          switch (ParseUnicodeGroup(&t, ps.flags(), re->ccb_, status)) {
+            case kParseOk:
+              if (!ps.PushRegexp(re))
+                return NULL;
+              goto Break2;
+            case kParseError:
+              re->Decref();
+              return NULL;
+            case kParseNothing:
+              re->Decref();
+              break;
+          }
+        }
+
+        UGroup *g = MaybeParsePerlCCEscape(&t, ps.flags());
+        if (g != NULL) {
+          Regexp* re = new Regexp(kRegexpCharClass, ps.flags() & ~FoldCase);
+          re->ccb_ = new CharClassBuilder;
+          AddUGroup(re->ccb_, g, g->sign, ps.flags());
+          if (!ps.PushRegexp(re))
+            return NULL;
+          break;
+        }
+
+        Rune r;
+        if (!ParseEscape(&t, &r, status, ps.rune_max()))
+          return NULL;
+        if (!ps.PushLiteral(r))
+          return NULL;
+        break;
+      }
+    }
+  Break2:
+    lastunary = isunary;
+  }
+  return ps.DoFinish();
+}
+
+}  // namespace re2