Regex: Emulate Atomic Grouping (and Possessive Quantifiers) with Lookahead

Some regex flavors (like JavaScript’s) do not support atomic grouping (nor possessive quantifiers), but fully support lookahead. Impractical, for atomic grouping usually improves performance and helps prevent catastrophic backtracking that can cause exponential matching time.

Fortunately, you can emulate Atomic Grouping with Lookahead: (?>a) becomes (?=(a))\1. It works by matching (and capturing) the sub-expression in a Lookahead assertion that doesn’t consume characters, then matching it (effectively consuming the characters) with a backreference, not allowing backtracking at this point.

As possessive quantifiers like a++ have a strict equivalent using atomic grouping (here (?>a+)), we can also leverage this technique: a++ becomes (?=(a+))\1.

There are two downsides though: it complexifies the regex, and it inserts a possibly unwanted capturing group for the sole purpose of matching. But this trick may still prove handy since the JS regex flavor is a bit subpar.

On a side note, REL implements this behavior when converting a regex for the JavaScript flavor.