[python] How to use a variable inside a regular expression?

I'd like to use a variable inside a regex, how can I do this in Python?

TEXTO = sys.argv[1]

if re.search(r"\b(?=\w)TEXTO\b(?!\w)", subject, re.IGNORECASE):
    # Successful match
else:
    # Match attempt failed

This question is related to python regex python-3.x variables escaping

The answer is


more example

I have configus.yml with flows files

"pattern":
  - _(\d{14})_
"datetime_string":
  - "%m%d%Y%H%M%f"

in python code I use

data_time_real_file=re.findall(r""+flows[flow]["pattern"][0]+"", latest_file)

I needed to search for usernames that are similar to each other, and what Ned Batchelder said was incredibly helpful. However, I found I had cleaner output when I used re.compile to create my re search term:

pattern = re.compile(r"("+username+".*):(.*?):(.*?):(.*?):(.*)"
matches = re.findall(pattern, lines)

Output can be printed using the following:

print(matches[1]) # prints one whole matching line (in this case, the first line)
print(matches[1][3]) # prints the fourth character group (established with the parentheses in the regex statement) of the first line.

if re.search(r"\b(?<=\w)%s\b(?!\w)" % TEXTO, subject, re.IGNORECASE):

This will insert what is in TEXTO into the regex as a string.


You have to build the regex as a string:

TEXTO = sys.argv[1]
my_regex = r"\b(?=\w)" + re.escape(TEXTO) + r"\b(?!\w)"

if re.search(my_regex, subject, re.IGNORECASE):
    etc.

Note the use of re.escape so that if your text has special characters, they won't be interpreted as such.


I find it very convenient to build a regular expression pattern by stringing together multiple smaller patterns.

import re

string = "begin:id1:tag:middl:id2:tag:id3:end"
re_str1 = r'(?<=(\S{5})):'
re_str2 = r'(id\d+):(?=tag:)'
re_pattern = re.compile(re_str1 + re_str2)
match = re_pattern.findall(string)
print(match)

Output:

[('begin', 'id1'), ('middl', 'id2')]

I agree with all the above unless:

sys.argv[1] was something like Chicken\d{2}-\d{2}An\s*important\s*anchor

sys.argv[1] = "Chicken\d{2}-\d{2}An\s*important\s*anchor"

you would not want to use re.escape, because in that case you would like it to behave like a regex

TEXTO = sys.argv[1]

if re.search(r"\b(?<=\w)" + TEXTO + "\b(?!\w)", subject, re.IGNORECASE):
    # Successful match
else:
    # Match attempt failed

rx = r'\b(?<=\w){0}\b(?!\w)'.format(TEXTO)

You can use format keyword as well for this.Format method will replace {} placeholder to the variable which you passed to the format method as an argument.

if re.search(r"\b(?=\w)**{}**\b(?!\w)".**format(TEXTO)**, subject, re.IGNORECASE):
    # Successful match**strong text**
else:
    # Match attempt failed

you can try another usage using format grammer suger:

re_genre = r'{}'.format(your_variable)
regex_pattern = re.compile(re_genre)  

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