Carets (^) in regular expressions

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Alan - 06 Oct 2007 03:27 GMT

The Java SE 6.0 API documentation for the Pattern class in
java.util.regex says the caret ("^") can be used in a regular
expression to find the beginning of a line. However, it can also be
used to denote negation (see "Character classes" section). How does
Java tell the difference in this usage?

Thanks, Alan

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Arne Vajhøj - 06 Oct 2007 03:35 GMT

> The Java SE 6.0 API documentation for the Pattern class in
> java.util.regex says the caret ("^") can be used in a regular
> expression to find the beginning of a line. However, it can also be
> used to denote negation (see "Character classes" section). How does
> Java tell the difference in this usage?

Java regex does what other regex does: ^ outside [] is start and
^ inside [] is negation.

Arne

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Alan - 06 Oct 2007 04:04 GMT

Arne,
Thanks. This is the first regexp I`ve used. It turns out my
problem was using "^\p" instead of the correct "\P".

Alan

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Roedy Green - 06 Oct 2007 04:29 GMT

> Thanks. This is the first regexp I`ve used. It turns out my
>problem was using "^\p" instead of the correct "\P".

for other regex hints, see http://mindprod.com/jgloss/regex.html

Signature

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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Arne Vajhøj - 06 Oct 2007 04:46 GMT

> Thanks. This is the first regexp I`ve used. It turns out my
> problem was using "^\p" instead of the correct "\P".

You could also have used ^ just inside [].

Try running:

import java.util.regex.Pattern;

public class R {
public static void main(String[] args) {
System.out.println(Pattern.matches("\\p{Digit}+", "123"));
System.out.println(Pattern.matches("\\p{Digit}+", "ABC"));
System.out.println(Pattern.matches("\\P{Digit}+", "123"));
System.out.println(Pattern.matches("\\P{Digit}+", "ABC"));
System.out.println(Pattern.matches("[^\\p{Digit}]+", "123"));
System.out.println(Pattern.matches("[^\\p{Digit}]+", "ABC"));
}
}

Arne

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Carets (^) in regular expressions

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