Python Regex Lookbehind

Summary: in this tutorial, you’ll learn about Python regex lookbehind and negative lookbehind.

Introduction to the Python regex lookbehind #

In regular expressions, the lookbehind matches an element if there is another specific element before it. The lookbehind has the following syntax:

(?<=Y)X

In this syntax, the pattern will match X if there is Y before it.

For example, suppose you have the following string and want to match the number 500 not the number 1:

'1 phone costs $500'Code language: JavaScript (javascript)

To do that, you can use the following regular expression with a lookahead like this:

(?<=\$)\d+

In this pattern:

• (?<=\$) matches an element if there is a literal string $ before it. Since the $ is a special character in the regex, we use the backslash character \ to escape it. As a result, the regex engine will treat \$ as a regular character $.
• \d+ matches one or more digits.

The following example uses a regular expression with a lookbehind to match a number that has the $ sign before it:

import re

s = '1 phone costs $500'
pattern = '(?<=\$)\d+'

matches = re.finditer(pattern, s)
for match in matches:
    print(match.group())
Code language: JavaScript (javascript)

Output:

500

Negative lookbehind #

The negative lookbehind has the following syntax:

(?<!Y)X

This pattern matches X if there is no Y before it.

The following example uses a negative lookbehind to match a number that doesn’t have the $ sign before it:

import re

s = '1 phone costs $500'
pattern = r'\b(?<!\$)\d+\b'

matches = re.finditer(pattern, s)
for match in matches:
    print(match.group())
Code language: JavaScript (javascript)

Output:

1

In the regular expression:

r'\b(?<!\$)\d+\b'Code language: JavaScript (javascript)

• The \b matches the word boundary.
• The (?<!\$) is a negative lookbehind that does not match the $ sign.
• The \d+ matches a number with one or more digits.

Summary #

• A lookbehind (?<!Y)X matches X only if there is element Y before it.
• A negative lookbehind (?<!Y)X matches X only if there’s no element Y before it.