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.
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Characters
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B
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The specific character B
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\xhh
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Character with hex value 0xhh
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\uhhhh
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The Unicode character with hex representation 0xhhhh
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\t
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Tab
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\n
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Newline
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\r
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Carriage return
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\f
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Form feed
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\e
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Escape
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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:
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Character Classes
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.
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Represents any character
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[abc]
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Any of the characters a, b, or c (same as a|b|c)
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[^abc]
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Any character except a, b, and c (negation)
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[a-zA-Z]
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Any character a through z or A through Z (range)
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[abc[hij]]
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Any of a,b,c,h,i,j (same as a|b|c|h|i|j) (union)
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[a-z&&[hij]]
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Either h, i, or j (intersection)
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\s
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A whitespace character (space, tab, newline, formfeed, carriage return)
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\S
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A non-whitespace character ([^\s])
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\d
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A numeric digit [0-9]
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\D
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A non-digit [^0-9]
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\w
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A word character [a-zA-Z_0-9]
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\W
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A non-word character [^\w]
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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.
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Logical Operators
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XY
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X followed by Y
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X|Y
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X or Y
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(X)
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A capturing group. You can refer to the ith captured group later in the expression with \i
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Boundary Matchers
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^
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Beginning of a line
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$
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End of a line
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\b
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Word boundary
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\B
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Non-word boundary
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\G
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End of the previous match
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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.*
