Python: How to use RegEx in an if statement?

Question

I have the following code which looks through the files in one directory and copies files that contain a certain string into another directory, but I am trying to use Regular Expressions as the string could be upper and lowercase or a mix of both.

Here is the code that works, before I tried to use RegEx's

import os
import re
import shutil

def test():
    os.chdir("C:/Users/David/Desktop/Test/MyFiles")
    files = os.listdir(".")
    os.mkdir("C:/Users/David/Desktop/Test/MyFiles2")
    for x in (files):
        inputFile = open((x), "r")
        content = inputFile.read()
        inputFile.close()
        if ("Hello World" in content)
            shutil.copy(x, "C:/Users/David/Desktop/Test/MyFiles2")

Here is my code when I have tried to use RegEx's

import os
import re
import shutil

def test2():
    os.chdir("C:/Users/David/Desktop/Test/MyFiles")
    files = os.listdir(".")
    os.mkdir("C:/Users/David/Desktop/Test/MyFiles2")
    regex_txt = "facebook.com"
    for x in (files):
        inputFile = open((x), "r")
        content = inputFile.read()
        inputFile.close()
        regex = re.compile(regex_txt, re.IGNORECASE)

Im guessing that I need a line of code that is something like

if regex = re.compile(regex_txt, re.IGNORECASE) == True

But I cant seem to get anything to work, if someone could point me in the right direction it would be appreciated.

For future reference, you might find this useful: [Regular Expression HOWTO](http://docs.python.org/2/howto/regex.html) for Python 2.7. — detly, Jan 08 '13 at 23:16

score 151 · Accepted Answer · edited Dec 26 '22 at 21:07

151

import re
if re.match(regex, content):
  blah..

You could also use re.search depending on how you want it to match.

You can run this example:

"""
very nice interface to try regexes: https://regex101.com/
"""
# %%
"""Simple if statement with a regex"""
import re

regex = r"\s*Proof.\s*"
contents = ['Proof.\n', '\nProof.\n']
for content in contents:
    assert re.match(regex, content), f'Failed on {content=} with {regex=}'
    if re.match(regex, content):
        print(content)

edited Dec 26 '22 at 21:07

Raktim Biswas

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answered Jan 08 '13 at 23:11

aw4lly

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thanks, im assuming from your answer that if the regex didnt match anything it would return "None" – Jan 08 '13 at 23:14
Thats right. Check out http://docs.python.org/2/library/re.html#re.search to see the difference between using the search() vs the match() function. – aw4lly Jan 08 '13 at 23:20
6

@LWH91 if you wanted `.match()` then your code orginally should have been using `.startswith()` and not `in` :) – Jon Clements Jan 08 '13 at 23:30
7

Why not just `if re.match(regex, content): blah..`? – Little Bobby Tables May 05 '16 at 14:46
3

`re.match` can return `None` or an empty string `""` I can't test it currently but I believe checking explicitly for `None` will better allow you to test for the empty string case. – aw4lly May 07 '16 at 02:21
why use `is not None` ? doesn't `None` automatcally equals `False`? – Lei Yang Nov 14 '18 at 03:41
According to the documentation a match object is always truthy: https://docs.python.org/3/library/re.html#match-objects – Septatrix Feb 19 '21 at 13:41

Bob Stein · Answer 2 · 2023-02-25T16:54:26.700

`if re.search(r'pattern', string):`

Simple if-regex example:

if re.search(r'ing\b', "seeking a great perhaps"):     # any words end with ing?
    print("yes")

Complex if-regex example (pattern check, extract a substring, case insensitive):

search_object = re.search(r'^OUGHT (.*) BE$', "ought to be", flags=re.IGNORECASE)
if search_object:
    assert "to" == search_object.group(1)     # what's between ought and be?

Notes:

Use re.search() not re.match. The match method restricts to the start of the string, a confusing convention. If you want that, search explicitly with caret: re.search(r'^...', ...) (Or in re.MULTILINE mode use \A)
Use raw string syntax r'pattern' for the first parameter. Otherwise you would need to double up backslashes, as in re.search('ing\\b', ...)
In these examples, '\\b' or r'\b' is a special sequence meaning word-boundary for regex purposes. Not to be confused with '\b' or '\x08' backspace.
re.search() returns None if it doesn't find anything, which is always falsy.
re.search() returns a Match object if it finds anything, which is always truthy.
even though re.search() returns a Match object (type(search_object) is re.Match) I have taken to calling the return value a search_object. I keep returning to my own bloody answer here because I can't remember whether to use match or search. It's search, dammit.
a group is what matched inside pattern parentheses.
group numbering starts at 1.
Specs
Tutorial

This should have been the accepted answer because it was very thorough — Manny Fleurmond, Sep 13 '22 at 00:35

Mike Samuel · Answer 3 · 2013-01-08T23:17:39.810

The REPL makes it easy to learn APIs. Just run python, create an object and then ask for help:

$ python
>>> import re
>>> help(re.compile(r''))

at the command line shows, among other things:

search(...)

search(string[, pos[, endpos]]) --> match object or None. Scan through string looking for a match, and return a corresponding MatchObject instance. Return None if no position in the string matches.

so you can do

regex = re.compile(regex_txt, re.IGNORECASE)

match = regex.search(content)  # From your file reading code.
if match is not None:
  # use match

Incidentally,

regex_txt = "facebook.com"

has a . which matches any character, so re.compile("facebook.com").search("facebookkcom") is not None is true because . matches any character. Maybe

regex_txt = r"(?i)facebook\.com"

The \. matches a literal "." character instead of treating . as a special regular expression operator.

The r"..." bit means that the regular expression compiler gets the escape in \. instead of the python parser interpreting it.

The (?i) makes the regex case-insensitive like re.IGNORECASE but self-contained.

score 2 · Answer 4 · edited Mar 01 '13 at 00:19

First you compile the regex, then you have to use it with match, find, or some other method to actually run it against some input.

import os
import re
import shutil

def test():
    os.chdir("C:/Users/David/Desktop/Test/MyFiles")
    files = os.listdir(".")
    os.mkdir("C:/Users/David/Desktop/Test/MyFiles2")
    pattern = re.compile(regex_txt, re.IGNORECASE)
    for x in (files):
        with open((x), 'r') as input_file:
            for line in input_file:
                if pattern.search(line):
                    shutil.copy(x, "C:/Users/David/Desktop/Test/MyFiles2")
                    break

score 1 · Answer 5 · edited Mar 01 '13 at 00:19

1

Regex's shouldn't really be used in this fashion - unless you want something more complicated than what you're trying to do - for instance, you could just normalise your content string and comparision string to be:

if 'facebook.com' in content.lower():
    shutil.copy(x, "C:/Users/David/Desktop/Test/MyFiles2")

edited Mar 01 '13 at 00:19

Brad Larson

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answered Jan 08 '13 at 23:19

Jon Clements

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score 0 · Answer 6 · answered Jun 29 '22 at 17:02

With a running example:

"""
very nive interface to try regexes: https://regex101.com/
"""
# %%
"""Simple if statement with a regex"""
import re

regex = r"\s*Proof.\s*"
contents = ['Proof.\n', '\nProof.\n']
for content in contents:
    assert re.match(regex, content), f'Failed on {content=} with {regex=}'
    if re.match(regex, content):
        print(content)

Python: How to use RegEx in an if statement?

6 Answers6

`if re.search(r'pattern', string):`

`search(...)`

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