fix: handle unmatched optional capture groups in stateful lexer actions - #464
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When a rule has an optional capture group that does not match, regexp reports it with indices of -1. Building action groups from those indices panicked with a slice bounds error. Unmatched groups are now passed to actions as empty strings, which also prevents backreferences from shifting to the wrong group (issue alecthomas#324). Fixes alecthomas#324.
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When a stateful lexer rule has an optional capture group that does not match (e.g.
[a-z]([a-z])?matching"a"),regexp.FindStringSubmatchIndexreports it with indices of-1. Building the groups passed toActions sliced the input with those indices, panicking withslice bounds out of range.The reported
%x + ycase from #324 is a real-world instance: the keyword/Ident design pushes a state on%, and the next rule's action (Pop) crashed on the unmatched capture group.Changes:
lexer/stateful.go: unmatched capture groups are passed to actions as empty strings instead of panicking. Keeping the group position (rather than dropping it) also preserves the meaning of\nbackreferences to later states — an unmatched group genuinely captures the empty string.lexer/stateful_test.go: two new table cases — "UnmatchedOptionalCaptureGroup" (panics before this fix, per Panic when stateful lexer's non-Root rule has optional group but captures nothing #324) and "BackrefMatchesEmptyCaptureGroup" (exercises the parent-state group hand-off).Verification:
go test ./...(root +_examples) green; golangci-lint — no new issues.Fixes #324.