正则表达式 贪婪与非贪婪

The three kinds of regular expression quantifiers are greedy, reluctant, and possessive.

A greedy quantifier starts by looking at the entire string for a match. If no match is found, it eliminates
the last character in the string and tries again. If a match is still not found, the last character is again 
discarded and the process repeats until a match is found or the string is left with no characters. All the
quantifiers discussed to this point have been greedy.

A reluctant quantifier starts by looking at the first character in the string for a match. If that character
alone isn’t enough, it reads in the next character, forming a string of two characters. If still no match isfound, a reluctant quantifier continues to add characters from the string until either a match is found or
the entire string is checked without a match. Reluctant quantifiers work in reverse of greedy quantifiers.
A Possessive quantifier only tries to match against the entire string. If the entire string doesn’t produce a match, no further attempt is made. Possessive quantifiers are, in a manner of speaking, a one-shot deal.

 

What makes a quantifier greedy, reluctant, or possessive? It’s really all in the use of the asterisk, question
mark, and plus symbols. For example, the question mark alone (?) is greedy, but a question mark fol-
lowed by another question mark (??) is reluctant. To make the question mark possessive, append a plus
sign (?+). The following table shows all the greedy, reluctant, and possessive versions of the quantifiers
you’ve already learned.

Notes: Browser support for possessive quantifiers leaves much to be desired. Internet
Explorer and Opera don’t support possessive quantifiers and throw an error when
you try to use one. Mozilla won’t throw an error, but it treats possessive quantifiers
as greedy.