Advanced Lesson 1
Regular Expressions
Chapter 7: Groups
Substitutions
The true power of backreferences can be seen when you need to find and replace a string.
Let’s say you want to make the section headers in your LaTeX document to be chapters (perhaps you are converting your paper into a book?)
You can replace all instances of section with chapter, but keeping the original header titles using backreferences. (We’re omitting the backslashes from LaTeX for simplicity)
The function re.sub(pattern, replacement, string) or the method pattern.sub(replacement, string) of a Pattern object can be used for this. It is similar to the str.replace() method, except that you can also search for substrings using regular expressions.
>>> pattern = r"section{([^}]*)}"
>>> replacement = r"chapter{\1}"
>>> string = "section{Introduction} section{Literature review}"
>>> re.sub(pattern, replacement, string)
chapter{Introduction} chapter{Literature review}
As expected, you can also use named groups for this. You use \g<name> to refer to the named group in the original pattern. \g<1> works too (and is equivalent to \1).
>>> pattern = r"section{(?P<title>[^}]*)}"
>>> string = "section{Introduction} section{Literature review}"
>>> re.sub(pattern, r"chapter{\1}", string)
'chapter{Introduction} chapter{Literature review}'
>>> re.sub(pattern, r"chapter{\g<1>}", string)
'chapter{Introduction} chapter{Literature review}'
>>> re.sub(pattern, r"chapter{\g<title>}", string)
'chapter{Introduction} chapter{Literature review}'
There is also a re.subn() function that also returns the number of substrings replaced.