how to use a variable instead of a string inside regex search function in python

Question

I have this regex function to extract a specific word in a string

fileName = re.search(r'path1\w([A-Za-z\d]+)', self.fileList[0]).group(1)

path1 is an actual string

What if I would like to replace it by fileName variable where fileName = "path1"

I tried:

print re.search(r'\w([A-Za-z\d]+)' % fileName, self.fileList[0]).group(1)

I got this error:

TypeError: not all arguments converted during string formatting

Why did I get this error ? how to solve this problem

Answer 1

You need a %s in your regex :

print re.search(r'%s\w([A-Za-z\d]+)' % fileName, self.fileList[0]).group(1)

Or as a more pythoinc and flexible way you can use str.format function :

print re.search(r'{}\w([A-Za-z\d]+)'.format(fileName), self.fileList[0]).group(1)

Note that is second way if you have a list of file names you can loop over them and pass the file names to format .

Answer 2

One should be very careful when interpolating strings into languages like Regex. In this case, you should probably escape the string first:

expression = r'{}\w([A-Za-z\d]+)'.format(re.escape(fileName))
re.search(expression, self.fileList[0]).group(1)

It's maybe also worth noting regex 's named lists:

import regex

expression = regex.compile(r'\L<filename>\w([A-Za-z\d]+)', filename=[fileName])
expression.search(self.fileList[0]).group(1)

This avoids having to regex-escape the literal, and works better if there are multiple options. (Plus regex is tons better anyway, so all the more reason to use it!)