Xah Lee, 2005-02, 2011-01
This pages shows a simple example of using regex in Python ＆ Perl.
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.
See also: Pyhton Regex Documentation: String Pattern Matching.
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.