Python 3 Regex Issue

Question

Why does the following string1 regexp not match? I have tested it using this and it appears my regex-fu is accurate, so I figure I must be missing something in the python implementation:

import re
pattern = r".*W([0-9]+(\.5)?)[^\.]?.*$"
string1 = '6013-SFR6W4.5'
string2 = '6013-SFR6W4.5L'
print(re.match(pattern, string1)) # the return value is None
print(re.match(pattern, string2)) # this returns a re.match object

Here is a screenshot of an interactive session showing this issue.

EDIT

sys.version outputs 3.4.3

I, too, get results on both strings. What is the output of python --version? — Myk Willis
– Myk Willis, Commented Sep 19, 2015 at 1:10
Ok, the screenshot I uploaded shows a typo in my pattern, I had a "+" after the [^\.] should I delete this question? — wesanyer
– wesanyer, Commented Sep 19, 2015 at 1:15
What if you use: pattern = r"^.*W([0-9]+(\.5)?)[^\.]?.*$" ? Do you still get None? — jlnabais
– jlnabais, Commented Sep 19, 2015 at 1:17
Great! Going to update the answer, you're missing the ^ at the beginning of pattern definition. — jlnabais
– jlnabais, Commented Sep 19, 2015 at 1:20

Myk Willis · Accepted Answer · 2015-09-19 01:19:09Z

1

In the code you posted, you have:

pattern = r".*W([0-9]+(\.5)?)[^\.]?.*$"

But in the code from your screenshot, you have

pattern = r".*W([0-9]+(\.5)?)[^\.]+.*$"

(Note the ? near the end of the first pattern is replaced with a + in the second one)

answered Sep 19, 2015 at 1:19

Myk Willis

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Zev Chonoles · Accepted Answer · 2015-09-19 01:06:33Z

1

When I run the code you provided, I get return values on both:

$ python3 test.py
<_sre.SRE_Match object at 0x6ffffedc3e8>
<_sre.SRE_Match object at 0x6ffffedc3e8>

edited Sep 19, 2015 at 1:06

answered Sep 19, 2015 at 1:03

Zev Chonoles

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2 Comments

wesanyer Over a year ago

but it is followed by a "*" meaning that the "." must be matched "zero or more times". According to docs.python.org/3/library/re.html

Zev Chonoles Over a year ago

@wesanyer: Sorry, you're right. But now that I've tested it, it seems like your code works fine.

jlnabais · Accepted Answer · 2015-09-19 01:21:30Z

I've tried the exact same code and I have a match for both cases:

python3.4:

Python 3.4.3 (v3.4.3:9b73f1c3e601, Feb 23 2015, 02:52:03) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> pattern = r".*W([0-9]+(\.5)?)[^\.]?.*$"
>>> string1 = '6013-SFR6W4.5'
>>> print(re.match(pattern, string1))
<_sre.SRE_Match object; span=(0, 13), match='6013-SFR6W4.5'>
>>> string2 = '6013-SFR6W4.5L'
>>> print(re.match(pattern, string2))
<_sre.SRE_Match object; span=(0, 14), match='6013-SFR6W4.5L'>

python 2.7:

Python 2.7.6 (default, Sep  9 2014, 15:04:36) 
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> pattern = r".*W([0-9]+(\.5)?)[^\.]?.*$"
>>> string1 = '6013-SFR6W4.5'
>>> print(re.match(pattern, string1))
<_sre.SRE_Match object at 0x10abf83e8>
>>> string2 = '6013-SFR6W4.5L'
>>> print(re.match(pattern, string2))
<_sre.SRE_Match object at 0x10abf83e8>

Try use pattern = r"^.*W([0-9]+(\.5)?)[^\.]?.*$", with ^at the beginning.

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