September 20, 2008

Match-making for Java Strings

Posted in Software tagged , , , at 11:10 am by mj

(Inspired by Jeff Atwood’s recent ‘outing’ as a regex sympathizer, which got me thinking about the line between “too many” and “too few” regular expressions and how some languages make it a choice between “too few” and “none”.)

Java has a Pattern, which forces you to pre-declare your regex string.

And it has a Matcher, which matches on a String.

It should be noted that a Pattern‘s pattern turns most patterns into a mess of backslashes since the pattern is wrapped in a plain-old Java String.

So a Matcher has matches(), which matches using the Pattern‘s pattern, but only if the pattern would otherwise match the Matcher‘s whole string.

A Matcher can also find(), which matches a Pattern pattern even if the pattern would only match a substring of the String, which is what most patterns match and what most languages call matching on a string.

A Matcher can lookAt(), which matches on a Pattern pattern, which, like find(), can match a pattern on a substring of the string, but only if the String‘s matching substring starts at the beginning of the string.

The String matched by the Matcher can be sliced by a call to region(start,end), which allows matches() and lookAt() to interpret a substring of the String as being the whole string.

Now, after calling find() or any of Matcher‘s String-matching cousins, a consumer of a Matcher can call group(int) to get the String substring that the Matcher‘s Pattern‘s pattern captured when matching on the Matcher‘s String‘s string.

But if you’re lazy, and you have no groups in your pattern, and a Matcher‘s matches() is sufficient, then String gives you matches(pattern) which is precisely equivalent to constructing a Pattern with your pattern and passing a new Matcher your existing String!

So with effective use of Java object syntax, you too can use regular expressions to make your matches on Java Strings almost as obscurely as other languages clearly make matches on their strings!

Is it any wonder Java programmers don’t realize that regular expressions are a beautiful… thing?


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