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Regular Expressions Cookbook, 2nd Edition by Steven Levithan, Jan Goyvaerts

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2.3. Match One of Many Characters

Problem

Create one regular expression to match all common misspellings of calendar, so you can find this word in a document without having to trust the author’s spelling ability. Allow an a or e to be used in each of the vowel positions. Create another regular expression to match a single hexadecimal character. Create a third regex to match a single character that is not a hexadecimal character.

The problems in this recipe are used to explain an important and commonly used regex construct called a character class.

Solution

Calendar with misspellings

c[ae]l[ae]nd[ae]r
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Hexadecimal character

[a-fA-F0-9]
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Nonhexadecimal character

[^a-fA-F0-9]
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Discussion

The notation using square brackets is called a character class. A character class matches a single character out of a list of possible characters. The three classes in the first regex match either an a or an e. They do so independently. When you test calendar against this regex, the first character class matches a, the second e, and the third a.

Inside a character class, only four characters have a special function: \, ^, -, and ]. If you’re using Java or .NET, the opening bracket [ is also a metacharacter inside character classes.

A backslash always escapes the character ...

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