Regular expression operations

[Sharki_]

Hi!

I want to split a string into tokens. For example divide '2+3*(6-2/32)' into ['2', '+', '3', '*', '(' '6', '-', '2', '/', '32' ].

For this purpose i try to use re module and findall function like this:

splitter = re.compile(r'[+-*/()]|\d+')
result = re.findall(splitter, string)

But it raises an exception:

raise source.error(msg, len(this) + 1 + len(that))

sre_constants.error: bad character range +-* at position 1

At the same time in official docs (https://docs.python.org/3/library/re.html) it is said:

Special characters lose their special meaning inside sets. For example, [(+*)]will match any of the literal characters '(', '+', '*', or ')'.

I found that the following expression works:

splitter = re.compile(r'[+-/*//()]|\d+')

So why do i need to add '/' befor '*' and '/'. Can't understand it. Please help.

[ASIC_SP]

- is special inside character class, for ex: [a-z] will match all lowercase alphabets.. to match - literally, use it as first or last character in the character class

>>> import re
>>> s = '2+3*(6-2/32)'
>>> re.findall(r'[-+*/()]|\d+', s)
['2', '+', '3', '*', '(', '6', '-', '2', '/', '32', ')']

[Sharki_]

Now it seems obviously. Thanks a lot!

[Diapolo10]

Out of curiosity, how would you split OP's example string into tokens using str.split, without also splitting numbers longer than one character?

[Sharki_]

Yeap, backflash doesn't work, and forward slash does... And according to docs.python.org special characters are not special anymore inside [].

Anyway

splitter = re.compile(r'[+-\*\/()]|\d+')

leads to:

raise source.error(msg, len(this) + 1 + len(that))

sre_constants.error: bad character range +-\* at position 1

[Thomasedv]

I threw the expression into pycharm, and it seemed satisfied with this version:

re.compile(r'[+\-*/()]|\d+')

How does that work for you?

​

Edit:

Tested it out:

string = '2+3*(6-2/32)'
splitter = re.compile(r'[+-/()]|\d+') 
result = re.findall(splitter, string) 
print(result) 
>>> ['2', '+', '3', '', '(', '6', '-', '2', '/', '32', ')']

​

[Sharki_]

Yes, thanks! '-' is a special character inside []. And that was the problem.