REGEX atomic groups `(?>)`

By definition, a REGEX atomic group is a non-capturing group that exits the group and throws away all alternatives after the first match of the pattern inside the group, so backtracking is disallowed".

While a non-atomic group will allow backtracking, it will still find the first match, then if the matching ahead fails it will backtrack and find the next match, until a match for the entire expression is found or all possibilities are exhausted.

Example 1 : #

Let’s consider the input string: xoots.

• A non-atomic group in the expression (xoo|xoot)s applied to xoots will:

  • match its 1st alternative xoo, then fail as s does not immediately follow in xoot, and backtrack to its 2nd alternative;
  • match its 2nd alternative xoot, then succeed as s immediately follows in xoots, and stop.

• An atomic group in the expression (?>xoo|xoot)s applied to xoots will match its 1st alternative xoo, then fail as s does not immediately follow, and stop as backtracking is disallowed.

Example 2: #

Let’s consider the input string: bbabbbabbbbc.

• Without an atomic grouping for the regex pattern /(.*|b*)[ac]/, the string will have a single match which is the whole string, due to backtracking at the end to match [ac].

• In contrast, for the atomic pattern: /((?>.*)|b*)[ac]/, there will be only three matches to this regex, which are bba, bbba, bbbbc.