Mdthbs

Here is a practice exercise — Regex version of strip() $-$

Write a function that takes a string and does the same thing as the
strip() string method. If no other arguments are passed other than the
string to strip, then whitespace characters will be removed from the
beginning and end of the string. Otherwise, the characters specified
in the second argument to the function will be removed from the
string.

I have written the following code. Is there any better way to write it? Any feedback is highly appreciated.

import re



def regex_strip(s, chars = None):



    if chars == None:

        strip_left = re.compile(r'^s*')

        strip_right = re.compile(r's*$')



        s = re.sub(strip_left, "", s)

        s = re.sub(strip_right, "", s)

    else:

        strip_left = re.compile(r'^[' + re.escape(chars) + r']*')

        strip_right = re.compile(r'[' + re.escape(chars) + r']*$')

        s = re.sub(strip_left, "", s)   

        s = re.sub(strip_right, "", s)

    return s

Here is an example output -

s = '.*    alphabetatheta   *4453   +-'

print(regex_strip(s, '.+-*'))



>>>    alphabetatheta      *4453   

If you call regex_strip(s, ""), you will get:

re.error: unterminated character set at position 0

because neither ^[] nor []$ is a value regular expression. You could avoid this by using if not chars: instead of if chars == None:.

There is no need to re.compile() your regular expressions; you aren't saving the compiled patterns anywhere for re-use.

You can simplify your logic by using the reg-ex to capture the middle, non-stripped portion of the string, instead of doing two replacements for the start and end trim operations:

import re



def regex_strip(s, chars = None):



    if chars:

        trim = '[' + re.escape(chars) + ']*'

    else:

        trim = r's*'



    return re.fullmatch(f"{trim}(.*?){trim}", s).group(1)

I'm not sure the point of asking you to write your own strip() function is to delegate the task to the reg-ex engine. It seems like going out and buying a sledge hammer when the problem is to build a nut cracker.

DRY. Both branches do identical re.subs. Take them out:

    if chars == None:

        strip_left = re.compile(r'^s*')

        strip_right = re.compile(r's*$')

    else:

        strip_left = re.compile(r'^[' + re.escape(chars) + r']*')

        strip_right = re.compile(r'[' + re.escape(chars) + r']*$')

    s = re.sub(strip_left, "", s)   

    s = re.sub(strip_right, "", s)

    return s

I recommend to go one step further, and unify the computation of strip_*:

    if chars == None:

        chars = string.whitespace



    strip_left = re.compile(r'^[' + re.escape(chars) + r']*')

    strip_right = re.compile(r'[' + re.escape(chars) + r']*$')

    s = re.sub(strip_left, "", s)   

    s = re.sub(strip_right, "", s)

    return s

It is recommended to compare against None as chars is None rather than using ==.