Python regex not finding a number in the range of 1 to 34 as expected

Question

I'm trying to find a number which is in the range 1 to 34 in my string but not getting the expected output. Code:

re.findall("(\d[1-34])","hi34hi30hi12")

Output:

['34','12']

Where is 30 here?? Or am I doing it wrong??

`\d[1-34]` actually matches a digit, followed by a digit that's in the range 1-3, or an "4". — Maroun, Mar 15 '16 at 15:20

A. Rama · Answer 1 · 2016-03-15T15:48:20.363

1

That regex is wrong. As it's been said in a comment above, your regex will match a single digit \d, followed by a single character in the set {1,2,3,4} which is the explicit meaning of the character class you used [1-34]

This one matches all the 2 digit numbers from 00 to 34 :

re.findall("([0-2][0-9]|3[0-4])","hi34hi30hi12")

this expression is made of two parts : the first

[0-2][0-9]

matches two characters, the first be a 0, a 1 or a 2 and the second a numerical digit; the second part is alternative the the first match (using the | operator)

3[0-4]

and matches a 3 followed by a 0, a 1, a 2, a 3 or a 4.

That expression, thus, as required, matches all 2 digit numbers from 00 to 34.

edited Mar 15 '16 at 15:48

answered Mar 15 '16 at 15:23

A. Rama

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To really help OP, please explain why his regex is incorrect. – Maroun Mar 15 '16 at 15:40
The question asked for the range 1 to 34, not 00 to 34. – Anmol Singh Jaggi Mar 15 '16 at 15:52

Maroun · Answer 2 · 2016-03-15T15:30:23.260

0

\d[1-34] actually matches a digit, followed by a digit that's in the range 1-3, or an "4".

"34" was matched because \d matched 3, and then the 4 inside the character class matched the 4.
"12" was matched because, again, \d\ matched 1, and then 2 was matched because it's in the range 1-3.

As mentioned in the comments, a better solution would be matching all 2-digit numbers and verify the range manually:

>>> re.findall("(\d\d)","hi34hi30hi12")
['34', '30', '12']

Now iterate over the list and verify the range.

edited Mar 15 '16 at 15:30

answered Mar 15 '16 at 15:21

Maroun

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Thanks a lot! How can do my search then?? – Vishal Yarabandi Mar 15 '16 at 15:26
Do you mind explaining how the question marks work? – lochsh Mar 15 '16 at 15:28
Use `r"(? – Wiktor Stribiżew Mar 15 '16 at 15:29
Nope, that doesn't match 29 – A. Rama Mar 15 '16 at 15:29
@A.Rama correct. Will fix. – Maroun Mar 15 '16 at 15:30
@WiktorStribiżew why not using only `\d{2}`? – Maroun Mar 15 '16 at 15:31
@MarounMaroun: I love complicating things :) `\d{2}` will match any sequence of 2 digits. `(? – Wiktor Stribiżew Mar 15 '16 at 15:33
@WiktorStribiżew I guess I got a downvote for over simplifying :) – Maroun Mar 15 '16 at 15:34
I don't agree, the question was about the regex and not a programmatic solution. Your `\d\d\` regex is not a correct solution for the question. – A. Rama Mar 15 '16 at 15:35
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@A.Rama: Let me remark that a regex is also a "programmatic" solution. One should know how to use it in the specific code environment. – Wiktor Stribiżew Mar 15 '16 at 15:36
@A.Rama OP already got a regex for that. I suggested a better solution (at least for me). I hate using regex for all tasks in the word. For god sake, matching a range shouldn't be validated using a regex. – Maroun Mar 15 '16 at 15:36
Why using a regex when you just can parse the string yourself? That's because, when parsing large amount of data, a regex can be quite fast and powerful and expressive. Programmatic solutions or post-analysis are sometimes needed because regexs are not always able to express every possible 'word' that you want to match, but it's definitely not the case here. – A. Rama Mar 15 '16 at 15:41

Python regex not finding a number in the range of 1 to 34 as expected

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