Programming Perl, 4th Edition

Possessive Groups

As described in The Little Engine That /Could(n’t)?/, the Engine often backtracks as it proceeds through the pattern. You can block the Engine from backtracking back through a particular set of choices by creating a nonbacktracking subpattern. A possessive group looks like (?>PATTERN), and it works exactly like a simple noncapturing group (?:PATTERN), except that once PATTERN has found a match, it suppresses backtracking on any of the quantifiers or alternatives inside the subpattern. (Hence, it is meaningless to use this on a PATTERN that doesn’t contain quantifiers or alternatives.) The only way to get it to change its mind is to backtrack to something before the subpattern and reenter the subpattern from the left.

It’s like going into a car dealership. After a certain amount of haggling over the price, you deliver an ultimatum: “Here’s my best offer; take it or leave it.” If they don’t take it, you don’t go back to haggling again. Instead, you backtrack clear out the door. Maybe you go to another dealership and start haggling again. You’re allowed to haggle again, but only because you reentered the nonbacktracking pattern again in a different context.

For devotees of Prolog or SNOBOL, you can think of this as a scoped cut or fence operator.

Consider how in "aaab" =~ /(?:a*)ab/, the a* first matches three as, but then gives up one of them because the last a is needed later. The subgroup sacrifices some of what it wants in order for the whole match to succeed. (Which ...