Python Perl: Text Pattern Matching (regex) Example

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Xah Lee, 2005-02, 2011-01

This pages shows a simple example of using regex in Python ＆ Perl.

Python

Suppose you want to replace all strings of the form <img src="some.gif" width="30" height="20"> to <img src="some.png" width="30" height="20"> in your html files.

What you need, is to match a text pattern, and capture parts of it for replacement. This is called regular expression or regex. Python provides regex in its “re” module. Here's a example of how to use it in our case.

# -*- coding: utf-8 -*-
# Python

import re

text = r'''<p> look at this <img src="./some.gif" width="30" height="20"> pict
and this one <img class="xyz" src="../that.gif">, both are
beautiful, but also look: <img src ="my.gif">, and sequel
 <img src=
"girl.gif"> yeah! </p>'''

new = re.sub(r'src\s*=\s*"([^"]+)\.gif"', r'src="\1.png"', text)

print new

The first argument to “re.sub” is a regex pattern. The second argument is the replacement string, which can contain captured pattern (the \1) the third argument is the text to be checked. A optional 4th argument is number of replacement to make. If omitted, it replace all occurances of matches.

Perl

In Perl, regex replacement is done with s///. Example:

$text = "123";
$text =~ s/2/9/;
print $text;

If all you want is to test a match instead of replace, do like this: $text =~ m/regexPatternHere/. If there is a match, it returns true.