Can I define custom character class shorthands?

问题

Java provides some useful character classes like \d and \w. Can I define my own character classes? For example, it would be useful to be able to define shorthands for character classes like [A-Za-z_].

回答1:

Can I define my own character classes?

No, you can't.

Personally, when I have a (slightly) complicated regex, I break the regex up in smaller sub-regexes and then "glue" them together with a String.format(...) like this:

public static boolean isValidIP4(String address) {
    String block_0_255 = "(0|[1-9]\\d|2[0-4]\\d|25[0-5])";
    String regex = String.format(
            "%s(\\.%s){3}", 
            block_0_255, block_0_255
    );
    return address.matches(regex);
}

which is far more readable than a single pattern:

"(0|[1-9]\\d|2[0-4]\\d|25[0-5])(\\.(0|[1-9]\\d|2[0-4]\\d|25[0-5])){3}"

Note that this is just a quick example: validating IP addresses can probably better be done by a class from the java.net package, and if you'd do it like that, the pattern should be placed outside the method and pre-compiled.

Be careful with % signs inside your pattern!

回答2:

I suspect the closest you can get assuming you don't want to write your own complete regex engine is to write a string preprocessor that converts a string containing your shorthand symbols into another string with the symbols expanded.

回答3:

Yes and no. Yes, because [] is already a character class. No, you can not define custom "shortcuts" like \w. One reason is, that it were never JAVAs decision. These character classes are standardized somewhere else (POSIX?).

My 2 cent: Regular expressions are complex enough be itself. You shouldn't confuse yourself and others by introducing even more.

回答4:

I assume you're talking about using these custom shortcuts in regular expressions? If so, the answer is no, unless you write your own regular expression parser (which is something you don't want to do). Those shortcuts are part of a predefined spec that can't be altered.

来源：https://stackoverflow.com/questions/6775383/can-i-define-custom-character-class-shorthands

标签

java

regex

customization