feat: add matches_regex filter and string_matching_regex expression test

[craigayre]

Description

There's no direct way to test whether a string matches a regex. We've been working around it with things like:

{% if var and var|regex_replace('^pattern$', '') == '' %}

This adds a matches_regex filter and a string_matching_regex expression test so you can write:

{{ 'hello'|matches_regex('^[a-z]+$') }}

{% if var is string_matching_regex '[0-9]+' %}

{{ items|selectattr('name', 'string_matching_regex', '^foo') }}

Both use RE2 (com.google.re2j) consistent with the existing regex_replace filter. Both use partial matching (Matcher.find), anchor with ^...$ for full-string matching.

Testing

• MatchesRegexFilterTest (9 tests): matching/non-matching, anchored patterns, null input, null arg, missing arg, invalid regex, SafeString, and maxStringLength enforcement.
• IsStringMatchingRegexExpTestTest (5 tests): matching/non-matching, null arg, SafeString, and end-to-end selectattr filtering.

Notes

• Happy to change the naming (matches_regex / string_matching_regex) if there's something more appropriate.
• The implementations are based on the existing string_containing expression test and regex_replace filter. There could probably be some simplifications, but kept things consistent with the existing implementations intentionally.

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🤖 Claude Code assisted.