"Regular Expression" is a fancy way to say "pattern matcher." Humans can match patterns with relative ease. A machine has a bit more difficulty deciphering patterns, especially in text. As computing became more powerful, the methods for matching text grew into more flexible dialects.
Regular expressions can be one of the toughest concepts to grasp and use effectively in any programming language. Perl is no exception as its regular expressions engine is perhaps the most advanced regex engine in existence. Its power and flexibility also serve to confuse and intimidate many new comers. It is important to understand the Regular Expression engine as its often the cause of serious bottlenecks in programs of all shapes and sizes.