[ Thanks to David
Culp for this link. ]
“Many of the text processing GNU tools include a powerful
pattern matching mechanism called Regular Expressions. A more or
less complete implementation is supported by a utility called
“egrep”. Other text utilities such as “gawk” also support
regex’s. Some tools like “sed” and “grep” support an older less
powerful regex syntax. Perl implements a regex variant that has
proven so popular that the same syntax is used in Python
regex’s.”
“For the rest of this article we’ll be using the POSIX standard
regular expressions as supported by “egrep”. “egrep” is a tool that
searches for substrings.”