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Pattern Matching
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6.3. Matching Words

Problem

You want to pick out words from a string.

Solution

Think long and hard about what you want a word to be and what separates one word from the next, then write a regular expression that embodies your decisions. For example:

/\S+/               # as many non-whitespace bytes as possible
/[A-Za-z'-]+/       # as many letters, apostrophes, and hyphens

Discussion

Because words vary between applications, languages, and input streams, Perl does not have built-in definitions of words. You must make them from character classes and quantifiers yourself, as we did previously. The second pattern is an attempt to recognize "shepherd's" and "sheep-shearing" each as single words.

Most approaches will have limitations because of the vagaries of written human languages. For instance, although the second pattern successfully identifies "spank'd" and "counter-clockwise" as words, it will also pull the "rd" out of "23rd Psalm". If you want to be more precise when you pull words out from a string, you can specify the stuff surrounding the word. Normally, this should be a word-boundary, not whitespace:

/\b([A-Za-z]+)\b/            # usually best
/\s([A-Za-z]+)\s/            # fails at ends or w/ punctuation

Although Perl provides \w, which matches a character that is part of a valid Perl identifier, Perl identifiers are rarely what you think of as words, since we really mean a string of alphanumerics and underscores, but not colons or quotes. Because it's defined in terms of \w, \b may surprise you if you expect to match an English word boundary (or, even worse, a Swahili word boundary).

\b and \B can still be useful. For example, /\Bis\B/ matches the string "is" only within a word, not at the edges. And while "thistle" would be found, "vis--vis" wouldn't.

See Also

The treatment of \b, \w, and \s in perlre (1) and in the "Regular expression bestiary" section of Chapter 2 of Programming Perl; the words-related patterns in Recipe 6.23


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