Regular Expressions

Regular expressions are the core element of utilities like egrep (UNIX) and findstr (Windows NT), and of the Perl programming language.
They are available in many other programming languages too.
They are an extremely powerful mechanism for searching or manipulating text.
For example, compare the use of FIND and FINDSTR to find a date in a text string. Both are used to search "literal" strings withinin strings, most of the time.
I wouldn't know how to search for any (unknown) date with a FIND command (though it can be done using FOR /F), but FINDSTR's (limited) regular expressions extensions make it possible.

Getting Started

Read a book on regular expressions, a wide choice of titles has become available recently.
Personally I loved reading Mastering Regular Expressions, Second Edition by Jeffrey E.F. Friedl. A great book to really start mastering regular expressions.
Or if you want a quick start, try Ben Forta's Sams Teach Yourself Regular Expressions in 10 Minutes.

Nothing beats practice.

Download any of the Regular Expression Builder or Evaluator tools available and start playing.
Test my sample expressions.
Try to match all HTML tags in a file, or all outward links, or whatever you like.