regex-is-hard/README.md

Regular expressions are hard

Writing a high-quality implementation of POSIX Extended Regular Expressions does not seem easy. Ideally, the following features would be offered at the same time:

• Strict standards compliance (see https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html and https://pubs.opengroup.org/onlinepubs/9799919799/functions/regexec.html).
• Predictable performance (polynomial in the length of the inputs, and linear in the length of the matched string).
• Limited resource usage (memory, CPU time (also related to the previous point)).
• Absence of vendor-specific syntax extensions (for portability).

In practice, most implementations fall short of these goals, due to a variety of issues:

• Unclear wording in the standards, leading to diverging outcomes between implementations.
• Exponential matching time due to backtracking implementation (sometimes even mandated by non-standard syntax extensions).
• Excessive consumption of stack memory.
• Spurious matching failures (either indication of a non-match, or an error).
• Incorrect capturing behaviour of parenthesised subexpressions.

Here we test a small sample of regular expressions that are known to be hard to match properly against a couple of popular regex engines.

Run it

Build using Lix:

nix-build -A default # or libcxx, or musl

List all available engines:

result/bin/driver list

Check the specified engines (tries all if none is specified, not recommended on libstdc++ because std may crash):

result/bin/driver check [engine]…

Print the match results of the specified engines (tries all if none is specified, not recommended on libstdc++ because std may crash):

result/bin/driver results [engine]…

List of supported engines

• Boost.Regex (boost)
• C standard library (c)
• Oniguruma (oniguruma, does not claim POSIX compliance)
• PCRE2 (pcre, does not claim POSIX compliance)
• RE2 (re2, does not claim POSIX compliance)
• C++ standard library (std)
• TRE (tre)