Question about a weird Java regex behavior

Question

I'm working on a regex that could match the leading digits and . in a String. But it seems just not working correctly. The regex I'm using is

"^[\\.\\d]+"

Below is my code:

public void testMiscellaneous() throws Exception {
    System.out.println("~~~~~~~~~~~~~~~~~~~testMiscellaneous~~~~~~~~~~~~~~~~~~~~");
    String s1 = ".123 *[DP7_Dog]";
    String s2 = ".123";
    String s3 = "1.12.3";
    String s4 = "a1.12.3";
    final String numberRegex = "^[\\.\\d]+";
    System.out.println(s1.matches(numberRegex));
    System.out.println(s2.matches(numberRegex));
    System.out.println(s3.matches(numberRegex));
    System.out.println(s4.matches(numberRegex));
}

The outputs are

false
true
true
false

However, I would expect true, true, true, false. There must be something wrong with the regex, but I can't find it. Can anybody help? Thanks.

This doesn't cause the match to fail (see Carl's answer for that), but you don't have to escape the dot inside a character class. "[.\\d]" is enough (as a Java string). — Tim Pietzcker
– Tim Pietzcker, Commented Jul 29, 2010 at 16:04
Thanks, Tim. It works without the double escapes. When you say "a character class", is it the same as character set defined inside brackets? — Sapience
– Sapience, Commented Jul 29, 2010 at 16:27

Carl Smotricz · Accepted Answer · 2010-07-29 16:02:21Z

3

The problem is that matches() insists on matching your entire input String, as if there was a ^ at the beginning of your regexp and $ at the end.

You may be better off using Matcher.find() or Matcher.lookingAt(), or (if you want to be dumb and lazy like myself) simply tacking .* on the end of your pattern.

answered Jul 29, 2010 at 16:02

Carl Smotricz

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1 Comment

Sapience Over a year ago

Carl, thanks a lot for your answer. I append the .* at the end of regex, and it works. It confuses me because the split() method matches part of the input string. So, if I use the same regex in split() as used in matches(), it would give me an empty String[]. Thanks again for your instant help.

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