ICU-21492 Fix regex compile assertion failure. #1577
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A regex pattern containing nested look-behind blocks could trigger an assertion failure during pattern compilation. The problem was caused by an off-by-one error in the code that computes an upper bound on the match length, needed because look-behind expressions are constrained to not have unbounded match length. Nested look-behind blocks come into play because, when computing the maximum match length of an outer block, any inner look-behind blocks are skipped over - they do not directly contribute to the length matched by the outer block. The problem was in the code that skips over these nested look-behind blocks.
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Upstream PR: unicode-org/icu#1577 Upstream ticket: https://unicode-org.atlassian.net/browse/ICU-21492
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Upstream PR: unicode-org/icu#1577 Upstream ticket: https://unicode-org.atlassian.net/browse/ICU-21492
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A regex pattern containing nested look-behind blocks could trigger an assertion
failure during pattern compilation. The problem was caused by an off-by-one
error in the code that computes an upper bound on the match length, needed
because look-behind expressions are constrained to not have unbounded match
length.
Nested look-behind blocks come into play because, when computing the maximum
match length of an outer block, any inner look-behind blocks are skipped over -
they do not directly contribute to the length matched by the outer block. The
problem was in the code that skips over these nested look-behind blocks.
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