mruby-regexp: let an empty group take a quantifier - #7275
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An empty non-capturing group took no quantifier: `(?:)` followed by
`*`, `+`, `?` or an interval was refused with `target of repeat
operator is not specified`, where CRuby compiles the group as an empty
match and the quantifier as one of it. The scoped-option spelling
`(?i:)` and a plain `()` demoted by a named group were refused the same
way, while `()`, `(?:|)` and `(?=)` took the quantifier on both sides.
```ruby
/(?:)*/ =~ "" # CRuby: 0, mruby: RegexpError
/(?:){2}/ =~ "" # CRuby: 0, mruby: RegexpError
/a(?:)*b/ =~ "ab" # CRuby: 0, mruby: RegexpError
/(?i:)*/ =~ "" # CRuby: 0, mruby: RegexpError
/()*/ =~ "" # 0 in both
```
`compile_atom()` emits nothing for the empty group, so
`compile_quantified()` saw no atom and returned, leaving the quantifier
to `compile_seq()`, whose guard for a quantifier with no atom refused
it. `compile_atom()` now returns whether it read an atom, which an empty
group is and an option toggle `(?i)` is not, and `compile_quantified()`
reads the quantifiers of an atom that emitted nothing the way it reads
those of `a{0}`: with `skip_quantifiers()`, since a repeat of what
matches empty matches empty. `(?i)*` and a stray metacharacter still
reach the guard and are refused as CRuby refuses them.
📝 WalkthroughWalkthroughThe regexp compiler now treats empty groups as quantifiable empty-matching atoms. It preserves invalid-target errors for option toggles and other non-atom tokens. Syntax tests cover repetition forms, nesting, scoped options, braces, and error cases. ChangesRegexp empty-group quantifiers
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The change can incorrectly accept reversed interval quantifiers such as Possibly related PRs
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Inline comments:
In `@mrbgems/mruby-regexp/src/re_compile.c`:
- Around line 1823-1833: Update the shared parse_quantifier() implementation to
reject interval quantifiers when the minimum exceeds the maximum, ensuring
skip_quantifiers() does not consume invalid ranges such as {2,1} and compilation
still raises RegexpError. Add a regression test in the existing regexp syntax
tests covering Regexp.new("(?:){2,1}") and its expected error.
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mrbgems/mruby-regexp/src/re_compile.cmrbgems/mruby-regexp/test/regexp_syntax.rb
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An empty non-capturing group takes no quantifier:
(?:)followed by*,+,?or an interval raisestarget of repeat operator is not specified,where CRuby compiles the group as an empty match and the quantifier as one of
it. The scoped-option spelling
(?i:)and a plain()demoted by a namedgroup are refused the same way, while
(),(?:|)and(?=)take thequantifier on both sides:
compile_atom()emits nothing for the empty group, socompile_quantified()sees no atom and returns, and the quantifier is left to
compile_seq(), whoseguard for a quantifier with no atom refuses it.
()emits its twoRE_SAVEsand
(?:|)itsRE_SPLIT, so those have an atom.The fix
compile_atom()returns whether it read an atom, which an empty group is andan option toggle
(?i)is not, andcompile_quantified()reads thequantifiers of an atom that emitted nothing the way it already reads those of
a{0}, withskip_quantifiers(): a repeat of what matches empty matchesempty, so they emit nothing.
(?i)*and a stray metacharacter still reach theguard and are refused, as CRuby refuses them:
Size
.textofbin/mruby,build_config/ci/gcc-clang.rb, each side from a cleanbuild directory.
re_compile.ois the only object that changes: by 41 bytesat
-O0, and at-O3skip_quantifiers()has two callers now and gcc stopsinlining it into
compile_seq()(298 bytes inbintest), the rest being thelayout of
compile_seq(), into which the parser is inlined.bintestascii-ctypebyte-stringcxx_abifull-debug(-O0)Verification
The tests go in
regexp_syntax.rbbeside the non-capturing group test: thequantifiers above on
(?:),(?i:)and a demoted(), the lazy, possessiveand stacked spellings,
(?:) *under/x, a{that spells no quantifier,and
(?i)*still refused. On master the block fails at its first assertion,the literal being refused when it is compiled.
Differential against CRuby 4.0.6, the harness of #7269 and #7273 with
master and this PR against the same cases: 10,000 random patterns (seed 2,
the default features, over
aandb), each matched against one subject andcompared as
MatchData#to_a. Master refuses 1,657 of them, every one forthis reason, and CRuby cannot finish 1 under the memory cap. This PR refuses
none. Of the 1,657, 1,631 answer as CRuby does; 21 read a capture written
inside a lookaround, which is #7273 (with its fix stacked on this PR all 21
answer as CRuby does); and 5 are standing differences of the engine that the
empty group has no part in: each of the 5 answers the same, on master as
here, with its empty group and quantifier removed. The 8,342 cases both sides
compile answer the same on master and this PR, 100 of them differing from
CRuby on both.
rake test,build_config/ci/gcc-clang.rb, no compiler warning:full-debugbintestbintest(bintest suite)cxx_abibyte-stringascii-ctypeThe default configuration: 2,141 total, 0 KO, 0 crash, plus its 112 bintests.
Environment
Details
Compile lines for
mrbgems/mruby-regexp/src/re_compile.cin the buildsquoted above, paths shortened:
Summary by CodeRabbit
Bug Fixes
Tests