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Thinking in Java
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Creating regular expressions

You can begin learning regular expressions with a useful subset of the possible constructs. A complete list of constructs for building regular expressions can be found in the javadocs for the Pattern class for package java.util.regex.

Characters

B

The specific character B

\xhh

Character with hex value 0xhh

\uhhhh

The Unicode character with hex representation 0xhhhh

\t

Tab

\n

Newline

\r

Carriage return

\f

Form feed

\e

Escape

The power of regular expressions begins to appear when defining character classes. Here are some typical ways to create character classes, and some predefined classes:

Character Classes

.

Represents any character

[abc]

Any of the characters a, b, or c (same as a|b|c)

[^abc]

Any character except a, b, and c (negation)

[a-zA-Z]

Any character a through z or A through Z (range)

[abc[hij]]

Any of a,b,c,h,i,j (same as a|b|c|h|i|j) (union)

[a-z&&[hij]]

Either h, i, or j (intersection)

\s

A whitespace character (space, tab, newline, formfeed, carriage return)

\S

A non-whitespace character ([^\s])

\d

A numeric digit [0-9]

\D

A non-digit [^0-9]

\w

A word character [a-zA-Z_0-9]

\W

A non-word character [^\w]

If you have any experience with regular expressions in other languages, you’ll immediately notice a difference in the way backslashes are handled. In other languages, “\\” means “I want to insert a plain old (literal) backslash in the regular expression. Don’t give it any special meaning.” In Java, “\\” means “I’m inserting a regular expression backslash, so the following character has special meaning.” For example, if you want to indicate one or more word characters, your regular expression string will be “\\w+”. If you want to insert a literal backslash, you say “\\\\”. However, things like newlines and tabs just use a single backslash: “\n\t”.

What’s shown here is only a sampling; you’ll want to have the java.util.regex.Pattern JDK documentation page bookmarked or on your “Start” menu so you can easily access all the possible regular expression patterns.

Logical Operators

XY

X followed by Y

X|Y

X or Y

(X)

A capturing group. You can refer to the ith captured group later in the expression with \i


Boundary Matchers

^

Beginning of a line

$

End of a line

\b

Word boundary

\B

Non-word boundary

\G

End of the previous match

As an example, each of the following represent valid regular expressions, and all will successfully match the character sequence "Rudolph":

Rudolph
[rR]udolph
[rR][aeiou][a-z]ol.*
R.*


Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire