Atomic Groups and Possessive Quantifiers

Support at a glance

Engine	Atomic (?>...)	Possessive a++
Java	Yes	Yes
PHP / PCRE	Yes	Yes
Python re (3.11+)	Yes	Yes
Python regex module	Yes	Yes
JavaScript	No	No
Go (RE2)	No (not needed)	No (not needed)

What problem they solve

A pattern like (a+)+$ against a long string of as followed by a non-match can force the engine to try astronomically many combinations — this is catastrophic backtracking, and it can freeze a program for seconds or minutes. Atomic groups and possessive quantifiers tell the engine: "once you have matched this, never give any of it back." That collapses the search space and makes the failure fast instead of fatal.

Example

// Java — possessive + prevents backtracking blow-up
Pattern p = Pattern.compile("(a++)+$");   // fails fast, no hang
// Atomic-group equivalent:
Pattern.compile("(?>a+)+$");

If your engine lacks them

• JavaScript: no atomic groups — rewrite the pattern to avoid nested quantifiers, or emulate atomicity with a lookahead capture trick: (?=(a+))\1.
• Go: not applicable — RE2 cannot catastrophically backtrack in the first place, so it needs no possessive syntax.