Regular expression for simple math expressions

Question

As an exercise I was trying to come up with a regex to evaluate simple algebra like

q = \'23 * 345 - 123+65\'

From here I want to get \'23\', \

Answer 1

Simply try this.

import re
q = '23 * 345 - 123+65'
regexparse = r'(\d+)|[-+*/]'
for i in re.finditer(regexparse, q):
    print i.group(0)

output:

23
*
345
-
123
+
65

Answer 2

This is your regex:

(\d+\s*(\*|\/|\+|\-)\s*)+(\d+\s*)

(\d+\s*(\*|\/|\+|\-)\s*) will match the first part of your expression: 23 * and store * in the second group.

Then the + makes it repeat, but because repeating capture groups retain only their last match, it will discard 23 * and * and instead match 345 - and - in the second group.

The + works again on the next repeat to discard the last capture and instead capture 123+ in the first group and + in the second.

Next, + cannot repeat any more, so it stops, and (\d+\s*) starts matching to get 65.

---

The fact that repeating capture groups store only the last capture is how regex works by design and is like this in all regex engines AFAIK.

---

Further elaboration:

There's a difference between matching repeatedly and capturing repeatedly. Try: (\d)+ on 12345 and you will see that only 5 will be captured. It's like that because you the paren is assigned a particular group capture. The first group is assigned group 1 and if you have many captures for group 1, you can only keep 1 and that's the last. This is how regex works, unfortunately, as per the docs:

If a group matches multiple times, only the last match is accessible

---

If you want to get your desired output, you might use re.findall and match with \d+|[+/*-]:

import re
q = '23 * 345 - 123+65'
regexparse = r'\d+|[+/*-]'
elem = re.findall(regexparse, q)
print(elem)
#=> ['23', '*', '345', '-', '123', '+', '65']

Answer 3

Your regex is confusing. Better to use re.split() for this purpose:

q = '23 * 345 - 123+65'
print re.split('\s*([-+/*])\s*', q)

Outputs:

['23', '*', '345', '-', '123', '+', '65']

Answer 4

I can only speak of regex in general, as I don't know python, but your problem is that in

(\d+\s*[\*/+-]\s*)+(\d+\s*)

This portion

(\d+\s*[\*/+-]\s*)+

Is being repeated and when it's completely done evaluating, you only see the final one.