How can I check if a string contains a number between two brackets and return the location?

Question

Say I have str = "qwop(8) 5" and I want to return the position of 8.

I have the following solution:

import re

str = "qwop(8) 5"
regex = re.compile("\(\d\)")
match = re.search(regex, string) # match object has span = (4, 7)
print(match.span()[0] + 1)       # +1 gets at the number 8 rather than the first bracket

This seems really messy. Is there a more sophisticated solution? Preferably using re as I've already imported that for other uses.

Answer 1

Use match.start() to get the start index of the match, and a capturing group to capture specifically the digit between the brackets to avoid the +1 in the index. If you want the very start of the pattern, use match.start(), if you only want the digit, use match.start(1);

import re
test_str = 'qwop(8) 5'
pattern = r'\((\d)\)'
match = re.search(pattern, test_str)

start_index = match.start()
print('Start index:\t{}\nCharacter at index:\t{}'.format(start_index,
                                                         test_str[start_index]))
match_index = match.start(1)
print('Match index:\t{}\nCharacter at index:\t{}'.format(match_index,
                                                         test_str[match_index]))

Outputs;

Start index:    4
Character at index: (
Match index:    5
Character at index: 8

Answer 2

You can use:

regex = re.compile(r'\((\d+)\)')

The r prefix means that we are working with a raw string. A raw string means that if you write for instance r'\n', Python will not interpret this as a string with a new line character. But as a string with two characters: a backslash ('\\') and an 'n'.

The additional brackets are there to define a capture group. Furthermore a number is a sequence of (one or more) digits. So the + makes sure that we will capture (1425) as well.

We can then perform a .search() and obtain a match. You then can use .start(1) to obtain the start of the first capture group:

>>> regex.search(data)
<_sre.SRE_Match object; span=(4, 7), match='(8)'>
>>> regex.search(data).start(1)
5

If you are inteested in the content of the first capture group, you can call .group(1):

>>> regex.search(data).group(1)
'8'