正则表达式模式匹配

Question

I am using java regex library to find a pattern "String_OneOrMoreDigits". For example, "linenumber_1" or "linenumber_31" or "linenumber_456". I am trying the following pattern assuming I will get string of type "linenumber_2" or "linenumber_44". However, I just get the strings of type "linenumber_2", it does not match more than one number at the end. How to go about matching such strings?

Pattern pattern = Pattern.compile("(linenumber_[0-9])|(linenumber_[0-9][0-9])");

Answer 1

No need for an alternation, just use a "one or more" qualifier on the [0-9] :

Pattern pattern = Pattern.compile("linenumber_[0-9]+");

That will match "linenumber_1" and "linenumber_44" and "linenumber_12345984". If you only want to match one or two digits, you can do that by being more explicit about the number of digits allowed:

Pattern pattern = Pattern.compile("linenumber_[0-9]{1,2}");

Answer 2

to add on if you want specific amount of digits, instead of

Pattern pattern = Pattern.compile("linenumber_[0-9]{1,2}");

you could also use

Pattern pattern = Pattern.compile("linenumber_[0-9]{1|4|8}");

which would specify that you want either 1 digit, 4 digits or 8 digits. and like Crowder said,

Pattern pattern = Pattern.compile("linenumber_[0-9]+");

would match with any number of digits.

Answer 3

you can also use:

Pattern pattern = Pattern.compile("linenumber_\d+");

and if you need a case insensitive match, append (?i) at the start like this:

Pattern pattern = Pattern.compile("(?i)linenumber_\d+");

(?i) allows regex engine to perform case insensitive match. \\d would match any digit from 0 to 9