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58

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2

I don't understand why with this regex the method returns false;

Pattern.matches("\\bi", "an is");

the character i is at a word boundary!

+2  A: 

The whole string has to match if you use matches:

Pattern.matches(".*\\bi.*", "an is")

This allows 0 or more characters before and after. Or:

boolean anywhere = Pattern.compile("\\bi").matcher("an is").find();

will tell you if any substring matches (true in this case). As a note, compiling regexes then keeping them around can improve performance.

Matthew Flaschen
+1; I'll incorporate compiling patterns into my answer at some point.
polygenelubricants
+6  A: 

In Java, matches attempts to match a pattern against the entire string.

This is true for String.matches, Pattern.matches and Matcher.matches.

If you want to check if there's a match somewhere in a string, you can use .*\bi.*. In this case, as a Java string literal, it's ".*\\bi.*".

java.util.regex.Matcher API links


What .* means

As used here, the dot . is a regex metacharacter that means (almost) any character. * is a regex metacharacter that means "zero-or-more repetition of". So for example something like A.*B matches A, followed by zero-or-more of "any" character, followed by B (see on rubular.com).

References

Related questions

Note that both the . and * (as well as other metacharacters) may lose their special meaning depending on where they appear. [.*] is a character class that matches either a literal period . or a literal asterisk *. Preceded by a backslash also escapes metacharacters, so a\.b matches "a.b".


Related problems

Java does not have regex-based endsWith, startsWith, and contains. You can still use matches to accomplish the same things as follows:

  • matches(".*pattern.*") - does it contain a match of the pattern anywhere?
  • matches("pattern.*") - does it start with a match of the pattern?
  • matches(".*pattern") - does it end with a match of the pattern?

String API quick cheat sheet

Here's a quick cheat sheet that lists which methods are regex-based and which aren't:

polygenelubricants
where do you find the syntax of .* ??? Thanks
xdevel2000
You mentioned pretty much every string or regex method except [find](http://download.oracle.com/docs/cd/E17409_01/javase/7/docs/api/java/util/regex/Matcher.html#find%28%29). :)
Matthew Flaschen
@Matthew: yeah I specifically only list the ones in `java.lang.String`. I mean, I can write an essay if I really want to cover everything (e.g. compiling). I'm not sure if I really should, though.
polygenelubricants