| // Copyright 2008 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. |
| |
| // Determine whether this library should match PCRE exactly |
| // for a particular Regexp. (If so, the testing framework can |
| // check that it does.) |
| // |
| // This library matches PCRE except in these cases: |
| // * the regexp contains a repetition of an empty string, |
| // like (a*)* or (a*)+. In this case, PCRE will treat |
| // the repetition sequence as ending with an empty string, |
| // while this library does not. |
| // * Perl and PCRE differ on whether \v matches \n. |
| // For historical reasons, this library implements the Perl behavior. |
| // * Perl and PCRE allow $ in one-line mode to match either the very |
| // end of the text or just before a \n at the end of the text. |
| // This library requires it to match only the end of the text. |
| // * Similarly, Perl and PCRE do not allow ^ in multi-line mode to |
| // match the end of the text if the last character is a \n. |
| // This library does allow it. |
| // |
| // Regexp::MimicsPCRE checks for any of these conditions. |
| |
| #include "util/util.h" |
| #include "re2/regexp.h" |
| #include "re2/walker-inl.h" |
| |
| namespace re2 { |
| |
| // Returns whether re might match an empty string. |
| static bool CanBeEmptyString(Regexp *re); |
| |
| // Walker class to compute whether library handles a regexp |
| // exactly as PCRE would. See comment at top for conditions. |
| |
| class PCREWalker : public Regexp::Walker<bool> { |
| public: |
| PCREWalker() {} |
| bool PostVisit(Regexp* re, bool parent_arg, bool pre_arg, bool* child_args, |
| int nchild_args); |
| |
| bool ShortVisit(Regexp* re, bool a) { |
| // Should never be called: we use Walk not WalkExponential. |
| LOG(DFATAL) << "EmptyStringWalker::ShortVisit called"; |
| return a; |
| } |
| }; |
| |
| // Called after visiting each of re's children and accumulating |
| // the return values in child_args. So child_args contains whether |
| // this library mimics PCRE for those subexpressions. |
| bool PCREWalker::PostVisit(Regexp* re, bool parent_arg, bool pre_arg, |
| bool* child_args, int nchild_args) { |
| // If children failed, so do we. |
| for (int i = 0; i < nchild_args; i++) |
| if (!child_args[i]) |
| return false; |
| |
| // Otherwise look for other reasons to fail. |
| switch (re->op()) { |
| // Look for repeated empty string. |
| case kRegexpStar: |
| case kRegexpPlus: |
| case kRegexpQuest: |
| if (CanBeEmptyString(re->sub()[0])) |
| return false; |
| break; |
| case kRegexpRepeat: |
| if (re->max() == -1 && CanBeEmptyString(re->sub()[0])) |
| return false; |
| break; |
| |
| // Look for \v |
| case kRegexpLiteral: |
| if (re->rune() == '\v') |
| return false; |
| break; |
| |
| // Look for $ in single-line mode. |
| case kRegexpEndText: |
| case kRegexpEmptyMatch: |
| if (re->parse_flags() & Regexp::WasDollar) |
| return false; |
| break; |
| |
| // Look for ^ in multi-line mode. |
| case kRegexpBeginLine: |
| // No condition: in single-line mode ^ becomes kRegexpBeginText. |
| return false; |
| |
| default: |
| break; |
| } |
| |
| // Not proven guilty. |
| return true; |
| } |
| |
| // Returns whether this regexp's behavior will mimic PCRE's exactly. |
| bool Regexp::MimicsPCRE() { |
| PCREWalker w; |
| return w.Walk(this, true); |
| } |
| |
| |
| // Walker class to compute whether a Regexp can match an empty string. |
| // It is okay to overestimate. For example, \b\B cannot match an empty |
| // string, because \b and \B are mutually exclusive, but this isn't |
| // that smart and will say it can. Spurious empty strings |
| // will reduce the number of regexps we sanity check against PCRE, |
| // but they won't break anything. |
| |
| class EmptyStringWalker : public Regexp::Walker<bool> { |
| public: |
| EmptyStringWalker() { } |
| bool PostVisit(Regexp* re, bool parent_arg, bool pre_arg, |
| bool* child_args, int nchild_args); |
| |
| bool ShortVisit(Regexp* re, bool a) { |
| // Should never be called: we use Walk not WalkExponential. |
| LOG(DFATAL) << "EmptyStringWalker::ShortVisit called"; |
| return a; |
| } |
| |
| private: |
| DISALLOW_EVIL_CONSTRUCTORS(EmptyStringWalker); |
| }; |
| |
| // Called after visiting re's children. child_args contains the return |
| // value from each of the children's PostVisits (i.e., whether each child |
| // can match an empty string). Returns whether this clause can match an |
| // empty string. |
| bool EmptyStringWalker::PostVisit(Regexp* re, bool parent_arg, bool pre_arg, |
| bool* child_args, int nchild_args) { |
| switch (re->op()) { |
| case kRegexpNoMatch: // never empty |
| case kRegexpLiteral: |
| case kRegexpAnyChar: |
| case kRegexpAnyByte: |
| case kRegexpCharClass: |
| case kRegexpLiteralString: |
| return false; |
| |
| case kRegexpEmptyMatch: // always empty |
| case kRegexpBeginLine: // always empty, when they match |
| case kRegexpEndLine: |
| case kRegexpNoWordBoundary: |
| case kRegexpWordBoundary: |
| case kRegexpBeginText: |
| case kRegexpEndText: |
| case kRegexpStar: // can always be empty |
| case kRegexpQuest: |
| case kRegexpHaveMatch: |
| return true; |
| |
| case kRegexpConcat: // can be empty if all children can |
| for (int i = 0; i < nchild_args; i++) |
| if (!child_args[i]) |
| return false; |
| return true; |
| |
| case kRegexpAlternate: // can be empty if any child can |
| for (int i = 0; i < nchild_args; i++) |
| if (child_args[i]) |
| return true; |
| return false; |
| |
| case kRegexpPlus: // can be empty if the child can |
| case kRegexpCapture: |
| return child_args[0]; |
| |
| case kRegexpRepeat: // can be empty if child can or is x{0} |
| return child_args[0] || re->min() == 0; |
| } |
| return false; |
| } |
| |
| // Returns whether re can match an empty string. |
| static bool CanBeEmptyString(Regexp* re) { |
| EmptyStringWalker w; |
| return w.Walk(re, true); |
| } |
| |
| } // namespace re2 |