Matching an expression in python regex

Question

I'm trying to match a coordinate with a python string regex, but I'm not getting a result. I was to see if the input is a valid coordinate definition but I'm not getting the correct match using the code below. Can someone tell me what's wrong?

def coordinate(coord):
     a = re.compile("^(([0-9]+), ([0-9]+))$")
     b = a.match(coord)
     if b:
         return True
     return False

Currently, it's returning false even if I pass in (3, 4) which is a valid coordinate.

For a start, you want b = a.match(coord), not b = re.match(coord). — Tim Peters
– Tim Peters, Commented Nov 5, 2013 at 0:53
You need to escape some of the parentheses in the regex string. You have some parens in there and you mean literal parens but they will be interpreted as a capture group. — Mike Edwards
– Mike Edwards, Commented Nov 5, 2013 at 0:55
@TimPeters but then I'd expect a TypeError because of incorrect arguments... So I'm really not sure if we're dealing with the actual code here... — Jon Clements
– Jon Clements, Commented Nov 5, 2013 at 0:56
how do I escape the parens? I typed it wrong here, I used a.match in my program. Fixed. — Quanquan Liu
– Quanquan Liu, Commented Nov 5, 2013 at 0:56
If you're doing nothing more than returning True/False, then you can just do return bool(a.match(coord)) instead... However, it wouldn't hurt that much to return the match object anyway — Jon Clements
– Jon Clements, Commented Nov 5, 2013 at 1:02

score 3 · Accepted Answer · 2013-11-05 05:20:24Z

3

This works:

from re import match
def coordinate(coord):   
    return bool(match("\s*\(\s*-?\d+(?:\.\d+)?\s*,\s*-?\d+(?:\.\d+)?\s*\)\s*$", coord))

It is also quite powerful, having the ability to handle negative numbers, fractions, and an optional space between the numbers.

Below is a breakdown of the Regex pattern:

\s*         # Zero or more whitespace characters
\(          # An opening parenthesis
\s*         # Zero or more whitespace characters
-?          # An optional hyphen (for negative numbers)
\d+         # One or more digits
(?:\.\d+)?  # An optional period followed by one or more digits (for fractions)
\s*         # Zero or more whitespace characters
,           # A comma
\s*         # Zero or more whitespace characters
-?          # An optional hyphen (for negative numbers)
\d+         # One or more digits
(?:\.\d+)?  # An optional period followed by one or more digits (for fractions)
\s*         # Zero or more whitespace characters
\)          # A closing parenthesis
\s*         # Zero or more whitespace characters
$           # End of the string

edited Nov 5, 2013 at 5:20

answered Nov 5, 2013 at 1:06

user2555451

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4 Comments

aIKid Over a year ago

Maybe an optional space would be better. Great explanation!

Mark Reed Over a year ago

\s? seems odd to me - it matches one space, or one tab, or one newline, or any other single whitespace character, but not two spaces. Matching tabs but not multiple spaces feels inconsistent. If I were composing the regex, I would probably choose either \040? or \s*.

dawg Over a year ago

Consider an optional leading and ending space; i.e.: "\s*$-?\d+(?:\.\d+)?,\s?-?\d+(?:\.\d+)?$\s*$"

user2555451 Over a year ago

Thanks for the tips; I have incorporated them into my post. When building something complex, it sure does help to have it reviewed by more than just yourself. :)

blacktide · Accepted Answer · 2013-11-05 00:57:42Z

0

You need to escape the parentheses that you are trying to match. Try using the following:

^(\([0-9]+,\s[0-9]+\))$

answered Nov 5, 2013 at 0:57

blacktide

12.4k8 gold badges42 silver badges59 bronze badges

Comments

jlr · Accepted Answer · 2013-11-05 00:59:30Z

0

It seems to me that you do not properly escape the parenthesis that makes the coordinate syntax. In regex, parenthesis are special characters used for grouping and capturing. You need to escape them this way:

>>> a = re.compile("^\(([0-9]+), ([0-9]+)\)$")
>>> a.match("(3, 4)")
<_sre.SRE_Match object at 0x0000000001D91E00>

answered Nov 5, 2013 at 0:59

jlr

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