Why are these regular expressions not working?

Question

I want a regex to match complex mathematical expressions. However I will ask for an easier regex because it will be the simplest case.

Example input: 1+2+3+4

I want to separate each char: [('1', '+', '2', '+', '3', '+', '4')]

With a restriction: there has to be at least one operation (i.e. 1+2).

My regex: ([0-9]+)([+])([0-9]+)(([+])([0-9]+))* or (\d+)(\+)(\d+)((\+)(\d+))*

Output for re.findall('(\d+)(\+)(\d+)((\+)(\d+))*',"1+2+3+4") :

[('1', '+', '2', '+4', '+', '4')]

Why is this not working? Is Python the problem?

This is not the way. What you really need is a 2 step. Validate test it for `^\d+(?:\+\d+)*$` then match with findall `\+|\d+`. Otherwise, if python doesn't support the `\G` anchor, you might as well just use split(). — , Jun 05 '17 at 22:33
And if you require at least 1 operation, test it with `^\d+\d+(?:\+\d+)*$` Just so you know, parsing _complex_ math expressions is quite a task. — , Jun 05 '17 at 22:34
Could you please @sln explain me how to do that 2 step? Thanks :) — Oscar Martinez, Jun 05 '17 at 22:48

score 3 · Accepted Answer · 2017-06-05T23:24:00.420

3

You could go the test route.
See if its valid using re.match
then just get the results with re.findall

Python code

import re

input = "1+2+3+4";
if re.match(r"^\d+\+\d+(?:\+\d+)*$", input) :
    print ("Matched")
    print (re.findall(r"\+|\d+", input))

else :
    print ("Not valid")

Output

Matched
['1', '+', '2', '+', '3', '+', '4']

edited Jun 05 '17 at 23:24

answered Jun 05 '17 at 23:14

score 0 · Answer 2 · answered Jun 05 '17 at 22:36

0

You need ([0-9]+)([+])([0-9]+)(?:([+])([0-9]+))*

you get the '+4' for the group is out the last two expressions (([+])([0-9]+)).

the ?: indicate to python dont get de string for this group in the output.

answered Jun 05 '17 at 22:36

Mauro

93
8

And why is that regex ignoring `+3`? – Oscar Martinez Jun 05 '17 at 22:39
1

Because this `(?:([+])([0-9]+))*` resets the capture groups on each quantified pass, that's why it can't be done this way... – Jun 05 '17 at 22:41

Why are these regular expressions not working?

2 Answers2