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[Ruby 3.3] Add more tests for Regexp.linear_time? #1344

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eregon merged 2 commits into from
Feb 4, 2026
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[Ruby 3.3] Add more tests for Regexp.linear_time? #1344

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Add more tests for Regexp.linear_time?
trinistr Feb 3, 2026
5cf429d
Expand Regexp.linear_time? specs and clearly categorize specs which o…
eregon Feb 4, 2026
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53 changes: 51 additions & 2 deletions core/regexp/linear_time_spec.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,56 @@
}.should complain(/warning: flags ignored/)
end

it "returns true for positive lookarounds" do
Regexp.linear_time?(/(?:(?=a*)a)*/).should == true
it "returns true for positive lookahead" do
Regexp.linear_time?(/a*(?:(?=a*)a)*b/).should == true
end

it "returns true for positive lookbehind" do
Regexp.linear_time?(/a*(?:(?<=a)a*)*b/).should == true
end

it "returns true for negative lookbehind" do
Regexp.linear_time?(/a*(?:(?<!a)a*)*b/).should == true
end

# There are two known ways to make Regexp linear:
# * Using a DFA (deterministic finite-state automaton) Regexp engine, which always matches in linear time (e.g. TruffleRuby with TRegex)
# * Caching position and state to avoid catastrophic backtracking (e.g. CRuby: https://bugs.ruby-lang.org/issues/19104)
#
# Both approach should be allowed and given that DFA Regexp engines
# are much faster there should be no specs preventing using them.
uses_regexp_caching = RUBY_ENGINE == 'ruby'
uses_dfa_regexp_engine = !uses_regexp_caching

# The following specs should not be relied upon,
# they are here only to illustrate differences between Regexp engines.
guard -> { uses_regexp_caching } do
it "returns true for negative lookahead" do
Regexp.linear_time?(/a*(?:(?!a*)a*)*b/).should == true
end

it "returns true for atomic groups" do
Regexp.linear_time?(/a*(?:(?>a)a*)*b/).should == true
end

it "returns true for possessive quantifiers" do
Regexp.linear_time?(/a*(?:(?:a)?+a*)*b/).should == true
end

it "returns true for positive lookbehind with capture group" do
Regexp.linear_time?(/.(?<=(a))/).should == true
end
end

# The following specs should not be relied upon,
# they are here only to illustrate differences between Regexp engines.
guard -> { uses_dfa_regexp_engine } do
it "returns true for non-recursive subexpression call" do
Regexp.linear_time?(/(?<a>a){0}\g<a>/).should == true
end

it "returns true for positive lookahead with capture group" do
Regexp.linear_time?(/x+(?=(a))/).should == true
end
end
end