PyMOTW: re - Regular Expressions

Regular expressions are text matching patterns described with a
formal syntax. The patterns are interpreted as a set of instructions,
which are then executed with a string as input to produce a matching
subset or modified version of the original. The term “regular
expressions” is frequently shortened to as “regex” or “regexp” in
conversation. Expressions can include literal text matching,
repetition, pattern-composition, branching, and other sophisticated
rules. A large number of parsing problems are easier to solve with a
regular expression than by creating a special-purpose lexer and
parser.

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