Python - How do I match this pattern (number/number) with regex?

Question

I am trying to get my head around the regex module in python. I tried to get my program to match the following pattern from a line of text that the user inputs:

a number between 8-13 "/" a number between 0-15

For example: 8/2, 11/13, 10/9, etc.

The pattern that I came up with was:

upstream = re.compile(r'[8-9|1[0-3][/][0-9|1[0-5]')

However, this regex works with mixed results:

Enter a slot/port : 8/2    
['8/2']                    # This is correct

Enter a slot/port : 1/0    
['1/0']                    # This should print the "else" statement

Enter a slot/port : 8/15
['8/1']                    # The output is incomplete

The problem seems to stem from the forward slash, but I am not sure. I do know that I need some assistance in solving this issue. If anyone can help me solve this, I would greatly appreciate it.

The complete script is below.

import re
pattern = re.compile(r'[8-9|1[0-3][/][0-9|1[0-5]')

upstream = input("Enter a slot/port : ")

if re.search((pattern), upstream):
    print(re.findall(pattern, upstream))
else:
    print("We have a problem")

Thanks in advance :)

Check out this website which helps test regex patterns for python. — 0x263A
– 0x263A, Commented Jul 19, 2021 at 22:39
Thanks, Wiktor! That works. Now I just have to continue reading the documentation to fully understand what is going on with the ? and !. :) — python_noob
– python_noob, Commented Jul 19, 2021 at 23:12
If you think a '/' might interfere with user input, it would also be reasonable to add two input() calls, one for the slot, and then one for the port. Remembering to include a / is probably not as important to you as the two numbers. — jorf.brunning
– jorf.brunning, Commented Jul 20, 2021 at 2:32
Do you have to use regex? It's trivially easy to do this using str.split() and then parsing the resulting strings into integers. — pho
– pho, Commented Jul 20, 2021 at 21:08

Ryszard Czech · Accepted Answer · 2021-07-20 21:13:11Z

Your expression is not well-formed, as you utilized square brackets where round brackets must be. [8-9|1[0-3] and [0-9|1[0-5] both are bad patterns as [8-9 and [0-9 are not closed character classes.

Use

\b(?:[89]|1[0-3])/(?:[0-9]|1[0-5])\b

See regex proof.

EXPLANATION

--------------------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
--------------------------------------------------------------------------------
  (?:                      group, but do not capture:
--------------------------------------------------------------------------------
    [89]                     any character of: '8', '9'
--------------------------------------------------------------------------------
   |                        OR
--------------------------------------------------------------------------------
    1                        '1'
--------------------------------------------------------------------------------
    [0-3]                    any character of: '0' to '3'
--------------------------------------------------------------------------------
  )                        end of grouping
--------------------------------------------------------------------------------
  /                        '/'
--------------------------------------------------------------------------------
  (?:                      group, but do not capture:
--------------------------------------------------------------------------------
    [0-9]                    any character of: '0' to '9'
--------------------------------------------------------------------------------
   |                        OR
--------------------------------------------------------------------------------
    1                        '1'
--------------------------------------------------------------------------------
    [0-5]                    any character of: '0' to '5'
--------------------------------------------------------------------------------
  )                        end of grouping
--------------------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char

Thank you so much for this great great response! Sorry that I took so long to answer, I wasn't aware that the question was reopened. Using the boundary really made a huge difference, since I wanted to ensure that useless input such as c8/9 would be ignored. I have marked your response as the answer.

mroussell · Accepted Answer · 2021-07-20 21:24:55Z

1

The regex you are using requires 1's on both sides of the "/", use the "|" symbol to imply OR statements such that there is a choice of "a or "b", "a|b". This would give you a regex more in line with "[8-9]|1[0-3]" for before the "/" and "[0-9]|1[0-5]" after. So in total, when using "(regex)" to group parts you want to be expressed separately, you could end up with a regex more inline with "([8-9]|1[0-3])/([0-9]|1[0-5])".

Hope this was helpful!

answered Jul 20, 2021 at 21:24

mroussell

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1 Comment

python_noob Over a year ago

Yes, it helped me a lot, thanks for responding. My problem was not understanding how to properly group the different subpatterns in order to get the desired result.

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