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Regular Expressions in Linux

What Are Regular Expressions?

A regular expression is a pattern template you define that a Linux utility Uses to filter text. A Linux utility (such as the sed editor or the awk program) matches the regular expression pattern against data as that data flows Into the utility. If the data matches the pattern, it's accepted for processing.

              If the data doesn't match the pattern, it's rejected. The regular expression pattern makes use of wildcard characters to represent one or more characters in the data stream.

Types of regular expressions:

There are two popular regular expression engines:

The POSIX Basic Regular Expression (BRE) engine

The POSIX Extended Regular Expression (ERE) engine

A.     Defining BRE Patterns:

The most basic BRE pattern is matching text characters in a data stream.