python regex pattern grabs digits in a minimal way

Question

I try to get miles and chains integer values from a string like "at (17.08)". The false decimal form of the input data is the choice of the data administrator. First I tried the following pattern on this string "17.08":

"((\d+)\.(\d\d))"

This behaved correctly:

group(0) is "17.08"
group(1) is "17.08"
group(2) is "17"
group(3) is "08"

but now for the "at (17.08) is a" variation: I want to be able to substitute a correctly formatted location for the "decimal" notation, with this pattern:

".*\(?((\d+)\.(\d\d))\)?.*"

When queried with re.match I get the following match groups:

group(0) is "(17.08)", OK.
group(1) is "7.08", where is the 1 going ?
group(2) is "7", where is the 1 going ?
group(3) is "08", still OK.

What am I doing wrong ? Why does "re" behave like this ? I have an idea this must be related to the "greedy"/"non-greedy" theme, but how ?

You're using capture groups, you should use search if you just intend to have one output. Use non capturing groups: (?:stuff) will make sure you don't capture subgroups within the regex itself. — Rocky Li
– Rocky Li, Commented Nov 26, 2018 at 16:43
What exactly are you try to extract? The 17, the 08, or 17.08? Also, on my machine, copying your pattern and example gives 7.08, not 17.08 as group 1. — iz_
– iz_, Commented Nov 26, 2018 at 16:46
@Tomothy32: I want to extract the 17 and the 08, but having the form 17.08 as well makes a string replace possible. Also, your try is indeed giving the correct answer for the second pattern. Sorry for not being clear. — Michiel Rademakers
– Michiel Rademakers, Commented Nov 26, 2018 at 16:52

adrtam · Accepted Answer · 2018-11-26 16:55:17Z

1

The reason is the .*\(? part at the prefix absorbed the 1 in the input, as the opening bracket is optional and .* is greedy. My way of solving the issue is using the following regex instead (note the space after first *):

".* \(?((\d+)\.(\d\d))\)?.*"

Assuming you always have a space before the opening bracket (if present) or the number.

answered Nov 26, 2018 at 16:55

adrtam

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1 Comment

Michiel Rademakers Over a year ago

Thanks a lot for this answer. It did the trick, and indeed the parenthesised form is always part of a comment, so that works out all right.

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