Java – * and * * differences

*What's the difference between and * *? Why** When using pattern Did not compile when compiling (". * *");?

Solution

See the Java quantifiers reference:

Greedy  Reluctant   Possessive  Meaning
X?      X??         X?+         X,once or not at all
X*      X*?         X*+         X,zero or more times
X+      X+?         X++         X,one or more times
X{n}    X{n}?       X{n}+       X,exactly n times
X{n,}   X{n,}?      X{n,}+      X,at least n times
X{n,m}  X{n,m}?     X{n,m}+     X,at least n but not more than m times

There is no * * quantifier When you use after, * or? (or even {n, m}), you can create possessive quantifiers (see the table above), but adding * quantifiers after * is considered a user error

That's why* Will match 0 characters (without pattern.dotall modifier) and. Except newline character** Throw an exception

Note that online regular expression testers will also warn you of this problem: dangling meta character '*' near index 2. * *^ (the same warning appears in ocpsoft regex tester)