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Regular Expressions as Patterns
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   A "regular expression", or "regexp", is a way of describing a class
of strings.  A regular expression enclosed in slashes (`/') is an `awk'
pattern that matches every input record whose text belongs to that
class.

   The simplest regular expression is a sequence of letters, numbers, or
both.  Such a regexp matches any string that contains that sequence.
Thus, the regexp `foo' matches any string containing `foo'.  Therefore,
the pattern `/foo/' matches any input record containing `foo'.  Other
kinds of regexps let you specify more complicated classes of strings.

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Regexp Usage
How to Use Regular Expressions
Regexp Operators
Regular Expression Operators
Case-sensitivity
How to do case-insensitive matching.