Engine

Python re runs in an isolated process with a 12-second deadline. JavaScript/PCRE syntax is not silently translated.

Groups

Unmatched optional groups are null; empty captures are empty strings. Zero-width matches use the engine iterator and cannot create an unbounded loop.

Writing guide

Use named groups for meaningful values and anchors for whole-input validation. Broad quantifiers can backtrack; warnings are advisory, not a ReDoS guarantee.

Inputs explained

Operation

Operation

Pattern

Python re syntax. Named groups use (?P<name>...). No delimiters are needed.

Flags

i: ignore case; m: multiline; s: dot-all; x: verbose; a: ASCII; u: Unicode. g means find all. JavaScript sticky y is not supported.

Test text

Up to 64 KiB. Offsets count Python Unicode code points, not UTF-16 units.

Replacement

Python replacement syntax: \1 or \g<name>. Preview only.

Test cases

Array of {input, expected: true/false, count: optional integer}.

Output

Output