JavaScript: The Good Parts: The Good Parts by Douglas Crockford

Author:Douglas Crockford [Crockford, Douglas]
Language: eng
Format: epub, pdf
Tags: COMPUTERS / Programming Languages / JavaScript
ISBN: 9780596158736
Publisher: O'Reilly Media
Published: 2008-05-08T07:00:00+00:00

Regexp Escape

The backslash character indicates escapement in regexp factors as well as in strings, but in regexp factors, it works a little differently.

As in strings, \f is the formfeed character, \n is the newline character, \r is the carriage return character, \t is the tab character, and \u allows for specifying a Unicode character as a 16-bit hex constant. In regexp factors, \b is not the backspace character.

\d is the same as [0-9]. It matches a digit. \D is the opposite: [^0-9].

\s is the same as [\f\n\r\t\u000B\u0020\u00A0\u2028\u2029]. This is a partial set of Unicode whitespace characters. \S is the opposite: [^\f\n\r\t\u000B\u0020\u00A0\u2028\u2029].

\w is the same as [0-9A-Z_a-z]. \W is the opposite: [^0-9A-Z_a-z]. This is supposed to represent the characters that appear in words. Unfortunately, the class it defines is useless for working with virtually any real language. If you need to match a class of letters, you must specify your own class.

A simple letter class is [A-Za-z\u00C0-\u1FFF\u2800-\uFFFD]. It includes all of Unicode’s letters, but it also includes thousands of characters that are not letters. Unicode is large and complex. An exact letter class of the Basic Multilingual Plane is possible, but would be huge and inefficient. JavaScript’s regular expressions provide extremely poor support for internationalization.

\b was intended to be a word-boundary anchor that would make it easier to match text on word boundaries. Unfortunately, it uses \w to find word boundaries, so it is completely useless for multilingual applications. This is not a good part.

\1 is a reference to the text that was captured by group 1 so that it can be matched again. For example, you could search text for duplicated words with:

var doubled_words = /([A-Za-z\u00C0-\u1FFF\u2800-\uFFFD]+)\s+\1/gi;

doubled_words looks for occurrences of words (strings containing 1 or more letters) followed by whitespace followed by the same word.

\2 is a reference to group 2, \3 is a reference to group 3, and so on.

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