Regex Quantifiers

Quantifiers specify how often the preceding regular expression construct should match. The classes of quantifiers are:

The difference between greedy, reluctant, and possessive quantifiers involves what part of the string to try for the initial match, and how to retry if the initial attempt doesn't produce a match.

By default, quantifiers are

greedy

. A greedy quantifier first tries for a match with the entire input string. If that produces a match, then it considers the match a success and the engine can move on to the next construct in the regular expression. If the first try doesn't produce a match, the engine backs-off 1 character at a time until it finds a match. So a greedy quantifier checks for possible matches in order from the longest possible input string to the shortest possible input string, recursively trying from right to left.

Adding a

?

(question mark) to a greedy quantifier makes it

reluctant

. A reluctant quantifier first tries for a match from the beginning of the input string, starting with the shortest possible piece of the string that matches the regex construct. If that produces a match, then it considers the match a success and the engine can move on to the next construct in the regular expression. If the first try doesn't produce a match, the engine adds 1 character at a time until it finds a match. So a reluctant quantifier checks for possible matches in order from the shortest possible input string to the longest possible input string, recursively trying from left to right.

Adding a

+

(plus sign) to a greedy quantifier makes it

possessive

. A possessive quantifier is like a greedy quantifier on the first attempt (it tries for a match with the entire input string). The difference is that unlike a greedy quantifier, a possessive quantifier doesn't retry a shorter string if it doesn't find a match. If the initial match fails, the possessive quantifier reports a failed match. It doesn't make any more attempts.