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.

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").

Source: Wikipedia

Python regular expression syntax follows in the Perl lineage. The Python module re provides regular expression functionality. Regular Expressions are a sub-language embedded within the larger Python language.
 

Educational Resources

Python for Informatics

• 
  Chapter 11 - Regular Expressions (Slides, Printable Slides, Download Video, Streaming Video)

Dive Into Python 3

• 
  Chapter 5 - Regular Expressions

Python.org

• Regular Expression HOWTO

Practice

Please post regex excercises and questions below. We can help each other learn and explore this robust and slightly difficult aspect of Python.