Why is the return value of an empty python regexp search a match?

Question

When passing an empty string to a regular expression object, the result of a search is a match object an not None. Should it be None since there is nothing to match?

import re

m = re.search("", "some text")
if m is None:
    print "Returned None"
else:
    print "Return a match"

Incidentally, using the special symbols ^ and $ yield the same result.

score 12 · Accepted Answer · answered Dec 16 '10 at 20:18

12

Empty pattern matches any part of the string.

Check this:

import re

re.search("", "ffff")
<_sre.SRE_Match object at 0xb7166410>

re.search("", "ffff").start()
0

re.search("$", "ffff").start()
4

Adding $ doesn't yield the same result. Match is at the end, because it is the only place it can be.

answered Dec 16 '10 at 20:18

gruszczy

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if no matches are found, what does it return? Some falsy value like `None` or does it throw an exception?! [nevermind, found my answer in the documentation.](https://docs.python.org/2/library/re.html#re.search) – oldboy Jul 18 '18 at 20:33

score 3 · Answer 2 · answered Dec 16 '10 at 20:20

3

Look at it this way: Everything you searched for was matched, therefore the search was successful and you get a match object.

answered Dec 16 '10 at 20:20

sth

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score 1 · Answer 3 · answered Dec 16 '10 at 20:18

1

What you need to be doing is not checking if m is None, rather you want to check if m is True:

if m:
    print "Found a match"
else:
    print "No match"

Also, the empty pattern matches the whole string.

answered Dec 16 '10 at 20:18

Rafe Kettler

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If m is `None` then `bool(m)` is `False`. He should check, whether `m` is `None`. – gruszczy Dec 16 '10 at 20:22
@gruszczy that doesn't say if there's a match, though – Rafe Kettler Dec 16 '10 at 20:23
In [5]: bool(re.search("", "ffff")) Out[5]: True In [6]: bool(re.search("$", "ffff")) Out[6]: True – gruszczy Dec 16 '10 at 20:25
1.) an empty pattern matches anything, 2.) `bool(re.match('$', 'ffff'))` returns False. The searching behavior must be some regexp quirk – Rafe Kettler Dec 16 '10 at 20:29
Of course it returns False, because `re.match('$', 'ffff')` returned None, which gives False when coerced to bool. – gruszczy Dec 16 '10 at 20:35

score 1 · Answer 4 · answered Dec 16 '10 at 20:19

1

Those regular expressions are successfully matching 0 literal characters.

All strings can be thought of as containing an infinite number of empty strings between the characters:

'Foo' = '' + '' + ... + 'F' + '' + ... + '' + 'oo' + '' + ...

answered Dec 16 '10 at 20:19

Cameron

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Why is the return value of an empty python regexp search a match?

4 Answers4

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