In computing, a regular expression, also referred to as regex or regexp, provides a concise and flexible means for matching strings of text, such as particular characters, words, or patterns of characters. A regular expression is written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.

The following examples illustrate a few specifications that could be expressed in a regular expression:

• The sequence of characters "car" appearing consecutively in any context, such as in "car", "cartoon", or "bicarbonate"
• The sequence of characters "car" occurring in that order with other characters between them, such as in "Icelander" or "chandler"
• The word "car" when it appears as an isolated word
• The word "car" when preceded by the word "blue" or "red"
• The word "car" when not preceded by the word "motor"
• A dollar sign immediately followed by one or more digits, and then optionally a period and exactly two more digits (for example, "$100" or "$245.99").

Regular expressions can be much more complex than these examples.

The rest of the article can be found on wikipedia with some nice examples.