正则表达式量词和字符类

Question

There are examples and descriptions of regex quantifiers in Java Tutorial.

Greedy - eats full string then back off by one character and try again

 Regex: .*foo // greedy String to search: xfooxxxxxxfoo Found "xfooxxxxxxfoo" 

Reluctant - start at the beginning then eat one character at a time

 Regex: .*?foo // reluctant quantifier String to search: xfooxxxxxxfoo Found "xfoo", "xxxxxxfoo" 

Possessive - eats the whole string trying once for match

 Regex: .*+foo // possessive quantifier String to search: xfooxxxxxxfoo No match found 

They are ok and I understand them, but can someone explain to me what happens when regex is changed to the character class? Are there any other rules?

 Regex: [fx]* String to search: xfooxxxxxxfoo Found "xf","","","xxxxxxf","","","","" Regex: [fx]*? String to search: xfooxxxxxxfoo Found 15 zero-length matches Regex: [fx]*+ String to search: xfooxxxxxxfoo Found "xf","","","xxxxxxf","","","","" 

Answer 1

It applies the quantifier (greedy, reluctant/lazy, possessive) to the entire character class. This means it will match (greedily, lazily, etc) each literal character in the character class.

Regex: [fx]*
String to search: xfooxxxxxxfoo
Found "xf","","","xxxxxxf","","","",""

So it looks for zero or more of f or x . The engine finds xf which matches. It also matches on the empty string around the two o 's. It then matches the consecutive x 's because it's zero or more of f or x .

I would check out regex101.com for more detail on regexes, especially the debugger portion on the left sidebar