CODEWITHSUNDEEP
pythonregex
pranay reddy
pranay reddy

Reputation: 111

Python regex:combining re pattern format with a variable

I would like to combine a python variable and pattern. How can I do it?

below is what I would like to do.

re.search(r'**some_variable+pattern**',str_for_pattern_match,flags)

Thanks for your help.

Upvotes: 11

Views: 30002

Answers (5)

Paul
Paul

Reputation: 982

An update for using Python's f-string syntax (possibly the easiest of them all):

re.search(rf'**{some_variable}pattern**',str_for_pattern_match,flags)

Upvotes: 2

ekhumoro
ekhumoro

Reputation: 120638

You have to be careful when inserting strings into a regexp pattern.

This is because the string may contain special regexp characters which can lead to errors or give unexpected results.

To give an example:

>>> import re
>>> s = 'one*two*three*four*five'
>>> t = '*f'
>>> r = re.compile(r'%s\w+' % t)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.7/re.py", line 244, in _compile
    raise error, v # invalid expression
sre_constants.error: nothing to repeat

This fails because the inserted string contains *, which is a special regexp character.

However, this problem can be dealt with by using the re.escape function on the inserted string:

>>> r = re.compile(r'%s\w+' % re.escape(t))
>>> r.findall(s)
['*four', '*five']

Upvotes: 6

Ben
Ben

Reputation: 71505

Regular expression patterns are not some special extra thing that Python treats specially. A pattern is just a perfectly ordinary string value, which the re module will interpret as a pattern.

So the question isn't "how can I use a variable in a pattern?", but rather "how can I construct a string based on a variable?".

The Python docs have plenty of information on how to do this. Particularly useful will be the documentation on string methods. The most important of these for the purpose of constructing regular expressions is probably will probably be str.format (as demonstrated in eumiro's answer), which has a large section of its own describing how to format basic data types into template strings in almost any way you could desire.

If you can master the basic operations on strings, then sticking a variable into a regular expression will be the least of what you can do!

Upvotes: 7

lig
lig

Reputation: 3890

The usual string formatting way works well

re.search(r'**%s+pattern**' % some_variable, str_for_pattern_match, flags)

Upvotes: 16

eumiro
eumiro

Reputation: 212955

re.search(r'**{0}+pattern**'.format(variable_name), str_for_pattern_match, flags)

now all your {…} will be interpreted as string format placeholders.

Upvotes: 5

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