Programming Python, 3rd Edition

Regular Expression Pattern Matching

Splitting and joining strings is a simple way to process text, as long as it follows the format you expect. For more general text analysis tasks, Python provides regular expression matching utilities. Regular expressions are simply strings that define patterns to be matched against other strings. Supply a pattern and a string and ask whether the string matches your pattern. After a match, parts of the string matched by parts of the pattern are made available to your script. That is, matches not only give a yes/no answer, but also can pick out substrings as well.

Regular expression pattern strings can be complicated (let’s be honest—they can be downright gross to look at). But once you get the hang of them, they can replace larger handcoded string search routines—a single pattern string generally does the work of dozens of lines of manual string scanning code and may run much faster. They are a concise way to encode the expected structure of text and extract portions of it.

In Python, regular expressions are not part of the syntax of the Python language itself, but they are supported by extension modules that you must import to use. The modules define functions for compiling pattern strings into pattern objects, matching these objects against strings and fetching matched substrings after a match. They also provide tools for pattern-based splitting, replacing, and so on.

Beyond those generalities, Python’s regular expression story is complicated ...