Python regular expressions return true/false

Question

Using Python regular expressions how can you get a True/False returned? All Python returns is:

<_sre.SRE_Match object at ...>

try `bool(re.search(pattern=META_VAR_REGEX, string=coq_str))`. — Charlie Parker, Jul 21 '22 at 14:45

score 243 · Answer 1 · answered Jul 05 '11 at 00:10

243

If you really need True or False, just use bool

>>> bool(re.search("hi", "abcdefghijkl"))
True
>>> bool(re.search("hi", "abcdefgijkl"))
False

As other answers have pointed out, if you are just using it as a condition for an if or while, you can use it directly without wrapping in bool()

answered Jul 05 '11 at 00:10

John La Rooy

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`bool` value is needed when the conditional statement contains boolean arithmetic operation. eg: if (re.search("a","abc") & True): – Aizzat Suhardi Jun 24 '15 at 17:58
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@AizzatSuhardi, `&` is a **bitwise** operaration. `and` would be the **boolean** operation. – John La Rooy Nov 16 '15 at 04:44
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thanks @JohnLaRooy. correction: `(re.search("a","abc") and True)` – Aizzat Suhardi Nov 17 '15 at 19:22
how is this different from ignacio's answer? – Charlie Parker Feb 08 '17 at 03:21
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Wrapping in `bool` makes the intention of the programmer clear to the reader. – Guilherme Salomé Oct 09 '18 at 11:56
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This is not just clearer, in some cases necessary i.e. if you are using the assertTrue function in python unit test to check a regex match (you need a boolean value, not expression). Or any function where you need a boolean input... – DannyMoshe Feb 07 '19 at 23:06

score 178 · Accepted Answer · answered Jul 05 '11 at 00:01

178

Match objects are always true, and None is returned if there is no match. Just test for trueness.

if re.match(...):

answered Jul 05 '11 at 00:01

Ignacio Vazquez-Abrams

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re.match(...) would return true if the string's beginning part match the regular pattern. While search will confirm the pattern anywhere in the string. – Yu Shen Jun 08 '13 at 09:57
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It may be more desirable to use ``if re.match(...) is None:`` instead – NuclearPeon Feb 01 '17 at 22:42
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sorry, did you address the comments in your answer? Its unclear to me, do you mind clarifying? – Charlie Parker Feb 08 '17 at 03:20
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May I ask _why_ `re` is designed like this? If `match` objects are always true, why doesn't it just return `True` at the first place, given that we always need to know whether the answer is true or false anyway? – ytu May 24 '18 at 03:33
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@ytu: Because then you can do [everything else you need to](https://docs.python.org/3/library/re.html#match-objects). – Ignacio Vazquez-Abrams May 24 '18 at 03:34
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Note that `<_sre.SRE_Match... == True` and `<_sre.SRE_Match... == False` are both `False`, so you may wish to cast the result to a `bool` in some cases. – Pikamander2 Oct 03 '19 at 03:41
@NuclearPeon `re.match` will never return `False`, so is that actually useful? Or is it to cover situations where you're not sure what value might be passed in? – wjandrea Dec 27 '19 at 01:39
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Why not just convert to bool `if bool(re.match(...)):` – Shakeel Apr 18 '20 at 21:22

score 15 · Answer 3 · edited Aug 16 '19 at 20:33

15

Here is my method:

import re
# Compile
p = re.compile(r'hi')
# Match and print
print bool(p.match("abcdefghijkl"))

edited Aug 16 '19 at 20:33

Héctor Valverde

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answered Jul 10 '18 at 06:00

Vaibhav Desai

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p = re.comp**il**e(r'hi') :-P – Clay Aug 23 '18 at 21:35
yeah and it returns "False" – Ufos Oct 10 '18 at 10:31
if you change `match` to `search`, it'll return True – alofgran May 21 '20 at 21:35

score 11 · Answer 4 · edited Mar 20 '18 at 22:40

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Ignacio Vazquez-Abrams is correct. But to elaborate, re.match() will return either None, which evaluates to False, or a match object, which will always be True as he said. Only if you want information about the part(s) that matched your regular expression do you need to check out the contents of the match object.

edited Mar 20 '18 at 22:40

Xantium

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answered Jul 05 '11 at 00:04

cory

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score 8 · Answer 5 · answered Jul 05 '11 at 00:04

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One way to do this is just to test against the return value. Because you're getting <_sre.SRE_Match object at ...> it means that this will evaluate to true. When the regular expression isn't matched you'll the return value None, which evaluates to false.

import re

if re.search("c", "abcdef"):
    print "hi"

Produces hi as output.

answered Jul 05 '11 at 00:04

Gavin Anderegg

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You *always* get a return value; `None` is the default if nothing is explicitly returned. – Ignacio Vazquez-Abrams Jul 05 '11 at 00:05
Thanks -- corrected that. I was just going by what I saw in the REPL. – Gavin Anderegg Jul 05 '11 at 00:07

score 4 · Answer 6 · answered Oct 16 '19 at 05:30

You can use re.match() or re.search(). Python offers two different primitive operations based on regular expressions: re.match() checks for a match only at the beginning of the string, while re.search() checks for a match anywhere in the string (this is what Perl does by default). refer this

Python regular expressions return true/false

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