Cover by Douglas Crockford

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Chapter 7. Regular Expressions

Whereas the contrary bringeth bliss, And is a pattern of celestial peace. Whom should we match with Henry, being a king. . .

William Shakespeare, The First Part of Henry the Sixth

Many of JavaScript's features were borrowed from other languages. The syntax came from Java, functions came from Scheme, and prototypal inheritance came from Self. JavaScript's Regular Expression feature was borrowed from Perl.

A regular expression is the specification of the syntax of a simple language. Regular expressions are used with methods to search, replace, and extract information from strings. The methods that work with regular expressions are regexp.exec, regexp.test, string.match, string.replace, string.search, and string.split. These will all be described in Chapter 8. Regular expressions usually have a significant performance advantage over equivalent string operations in JavaScript.

Regular expressions came from the mathematical study of formal languages. Ken Thompson adapted Stephen Kleene's theoretical work on type-3 languages into a practical pattern matcher that could be embedded in tools such as text editors and programming languages.

The syntax of regular expressions in JavaScript conforms closely to the original formulations from Bell Labs, with some reinterpretation and extension adopted from Perl. The rules for writing regular expressions can be surprisingly complex because they interpret characters in some positions as operators, and in slightly different ...

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