Possessive Quantifiers

An quantifier in a regular expression may be greedy (the default), reluctant, or possesive. A possesive quantifier does this:

• The match starts with the first unmatched character in the string.
• The possessive quantifier starts matching from left to right one character at a time.
• The possessive quantifier matches as many characters as it can.
• Once a character has been matched, it remains in possession of the quantifier, even if this prevents other parts of the expression from matching.

An quantifier is made possessive by appending a plus sign. The expression X++ matches X one or more times, and never gives up what it has matched. For example, the expression

X++[A-Z]+

does match the string

XXXXZ

The X++ part of the expression eats up all the X's leaving the Z to be matched by the rest of the expression.

However, the expression does not match the string

XXXX

because the X++ eats the entire expression, and does not give up the last X for the code [A-Z]+ to match.

Java regular expressions support possessive quantifiers, but JavaScript does not, so there interactive applet on this page.

YYXXratsXX

([XY]++)rats\1