How to use a variable inside a regular expression

Question

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

You use string concatenation

Chris Eberle
– Chris Eberle

2011-08-03 18:00:45 +00:00
Commented Aug 3, 2011 at 18:00 — Chris Eberle
– Chris Eberle, Commented Aug 3, 2011 at 18:00

Ned Batchelder · Accepted Answer · 2011-08-03 18:05:54Z

359

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.

answered Aug 3, 2011 at 18:05

Ned Batchelder

378k77 gold badges583 silver badges675 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

deed02392 Over a year ago

What if your variable goes first? r'' + foo + 'bar' ?

OJFord Over a year ago

@deed02392 r'' not necessary if you do re.escape(foo), which you should anyway. Actually, I think re interprets whatever it's given as a unicode string regardless of whether you prefix r or not.

Praxiteles Over a year ago

Does .format() work as well in place of the re.escape or is re.escape() necessary?

Pedro Lobito Over a year ago

@praxiteles did u find the answer?

gdvalderrama Over a year ago

I'm not sure if this works in I need to have a group of which the variable is a part of. Other answers below look more intuitive for that, and don't break the regex into several expressions.

|

airborne · Accepted Answer · 2022-08-29 16:05:45Z

257

From python 3.6 on you can also use Literal String Interpolation, "f-strings". In your particular case the solution would be:

if re.search(rf"\b(?=\w){TEXTO}\b(?!\w)", subject, re.IGNORECASE):
    ...do something

EDIT:

Since there have been some questions in the comment on how to deal with special characters I'd like to extend my answer:

raw strings ('r'):

One of the main concepts you have to understand when dealing with special characters in regular expressions is to distinguish between string literals and the regular expression itself. It is very well explained here:

In short:

Let's say instead of finding a word boundary \b after TEXTO you want to match the string \boundary. The you have to write:

TEXTO = "Var"
subject = r"Var\boundary"

if re.search(rf"\b(?=\w){TEXTO}\\boundary(?!\w)", subject, re.IGNORECASE):
    print("match")

This only works because we are using a raw-string (the regex is preceded by 'r'), otherwise we must write "\\\\boundary" in the regex (four backslashes). Additionally, without '\r', \b' would not converted to a word boundary anymore but to a backspace!

re.escape:

Basically puts a backslash in front of any special character. Hence, if you expect a special character in TEXTO, you need to write:

if re.search(rf"\b(?=\w){re.escape(TEXTO)}\b(?!\w)", subject, re.IGNORECASE):
    print("match")

NOTE: For any version >= python 3.7: !, ", %, ', ,, /, :, ;, <, =, >, @, and ` are not escaped. Only special characters with meaning in a regex are still escaped. _ is not escaped since Python 3.3.(s. here)

Curly braces:

If you want to use quantifiers within the regular expression using f-strings, you have to use double curly braces. Let's say you want to match TEXTO followed by exactly 2 digits:

if re.search(rf"\b(?=\w){re.escape(TEXTO)}\d{{2}}\b(?!\w)", subject, re.IGNORECASE):
    print("match")

edited Aug 29, 2022 at 16:05

answered Apr 23, 2019 at 12:06

airborne

4,1854 gold badges18 silver badges31 bronze badges

13 Comments

Pedro Lobito Over a year ago

As of 2020, this is the simplest and most pythonic way to use a variable inside a regular expression

Jia Gao Over a year ago

This is definitely a WOW.

Harsha Reddy Over a year ago

can someone explain the significance of "rf" here

airborne Over a year ago

@HarshaReddy: 'r': This string is a raw string: If you don't use it, '\b' will be converted to the backspace character (docs.python.org/3/howto/regex.html#more-pattern-power). 'f' tells python that this is an 'f-string', s. link above, and enables you to write the variable into the curly braces-

PunchyRascal Over a year ago

How to write quantifiers in f-strings: fr"foo{{1,5}}" (double the braces)

|

Wiktor Stribiżew · Accepted Answer · 2017-09-22 07:01:12Z

55

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.

edited Sep 22, 2017 at 7:01

Wiktor Stribiżew

631k41 gold badges502 silver badges632 bronze badges

answered Aug 3, 2011 at 18:04

Bo Buchanan

7074 silver badges3 bronze badges

Comments

Wiktor Stribiżew · Accepted Answer · 2017-09-22 07:00:31Z

43

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

edited Sep 22, 2017 at 7:00

Wiktor Stribiżew

631k41 gold badges502 silver badges632 bronze badges

answered Aug 3, 2011 at 18:04

Cat Plus Plus

131k27 gold badges205 silver badges226 bronze badges

1 Comment

gdvalderrama Over a year ago

Won't this be a problem if I'm using {4} in my regex to express I want exactly 4 of whatever comes before?

Deepak Nagarajan · Accepted Answer · 2016-02-03 19:41:11Z

9

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')]

answered Feb 3, 2016 at 19:41

Deepak Nagarajan

1311 silver badge3 bronze badges

Comments

Pedro Lobito · Accepted Answer · 2017-12-09 02:45:44Z

6

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

edited Dec 9, 2017 at 2:45

Pedro Lobito

99.8k36 gold badges274 silver badges278 bronze badges

answered Mar 28, 2015 at 13:37

Max Carroll

4,8993 gold badges34 silver badges36 bronze badges

Comments

Kevin Chou · Accepted Answer · 2019-04-18 09:06:16Z

4

you can try another usage using format grammer suger:

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

answered Apr 18, 2019 at 9:06

Kevin Chou

5875 silver badges8 bronze badges

Comments

jdelaporte · Accepted Answer · 2015-10-23 20:43:37Z

2

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.

answered Oct 23, 2015 at 20:43

jdelaporte

1371 silver badge9 bronze badges

Comments

Ardhi · Accepted Answer · 2021-04-03 13:54:33Z

1

here's another format you can use (tested on python 3.7)

regex_str = r'\b(?<=\w)%s\b(?!\w)'%TEXTO

I find it's useful when you can't use {} for variable (here replaced with %s)

answered Apr 3, 2021 at 13:54

Ardhi

3,0452 gold badges26 silver badges31 bronze badges

Comments

Haneef Mohammed · Accepted Answer · 2018-11-08 04:40:50Z

0

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

edited Nov 8, 2018 at 4:40

answered Nov 7, 2018 at 10:49

Haneef Mohammed

1,18411 silver badges11 bronze badges

Comments

Pedro Lobito · Accepted Answer · 2020-02-20 21:20:03Z

-1

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)

edited Feb 20, 2020 at 21:20

Pedro Lobito

99.8k36 gold badges274 silver badges278 bronze badges

answered Feb 20, 2020 at 13:44

Nikolay Baranenko

1,7038 gold badges37 silver badges65 bronze badges

Collectives™ on Stack Overflow

How to use a variable inside a regular expression

11 Answers 11

7 Comments

13 Comments

Comments

1 Comment

Comments

Comments

Comments

Comments

Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

11 Answers 11

7 Comments

13 Comments

Comments

1 Comment

Comments

Comments

Comments

Comments

Comments

Comments

Comments

Linked

Related