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.
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
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.