Real Python: Regular Expressions: Regexes in Python

In this tutorial, you’ll explore regular expressions, also known as regexes, in Python. A regex is a special sequence of characters that defines a pattern for complex string-matching functionality.

Earlier in this series, in the tutorial Strings and Character Data in Python, you learned how to define and manipulate string objects. Since then, you’ve seen some ways to determine whether two strings match each other:

• You can test whether two strings are equal using the equality (==) operator.

• You can test whether one string is a substring of another with the in operator or the built-in string methods .find() and .index().

String matching like this is a common task in programming, and you can get a lot done with string operators and built-in methods. At times, though, you may need more sophisticated pattern-matching capabilities.

In this tutorial, you’ll learn:

• How to access the re module, which implements regex matching in Python
• How to use re.search() to match a pattern against a string
• How to create complex matching pattern with regex metacharacters

Fasten your seat belt! Regex syntax takes a little getting used to. But once you get comfortable with it, you’ll find regexes almost indispensable in your Python programming.

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Regexes in Python and Their Uses

Imagine you have a string object s. Now suppose you need to write Python code to find out whether s contains the substring '123'. There are at least a couple ways to do this. You could use the in operator:

>>>

>>> s = 'foo123bar'
>>> '123' in s
True

If you want to know not only whether '123' exists in s but also where it exists, then you can use .find() or .index(). Each of these returns the character position within s where the substring resides:

>>>

>>> s = 'foo123bar'
>>> s.find('123')
3
>>> s.index('123')
3

In these examples, the matching is done by a straightforward character-by-character comparison. That will get the job done in many cases. But sometimes, the problem is more complicated than that.

For example, rather than searching for a fixed substring like '123', suppose you wanted to determine whether a string contains any three consecutive decimal digit characters, as in the strings 'foo123bar', 'foo456bar', '234baz', and 'qux678'.

Strict character comparisons won’t cut it here. This is where regexes in Python come to the rescue.

A (Very Brief) History of Regular Expressions

In 1951, mathematician Stephen Cole Kleene described the concept of a regular language, a language that is recognizable by a finite automaton and formally expressible using regular expressions. In the mid-1960s, computer science pioneer Ken Thompson, one of the original designers of Unix, implemented pattern matching in the QED text editor using Kleene’s notation.

Since then, regexes have appeared in many programming languages, editors, and other tools as a means of determining whether a string matches a specified pattern. Python, Java, and Perl all support regex functionality, as do most Unix tools and many text editors.

The re Module

Regex functionality in Python resides in a module named re. The re module contains many useful functions and methods, most of which you’ll learn about in the next tutorial in this series.

For now, you’ll focus predominantly on one function, re.search().

re.search(<regex>, <string>)

Scans a string for a regex match.

re.search(<regex>, <string>) scans <string> looking for the first location where the pattern <regex> matches. If a match is found, then re.search() returns a match object. Otherwise, it returns None.

Read the full article at https://realpython.com/regex-python/ »

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