re.search clear solution for The Longest Palindromic by Serg900vd - python coding challenges

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re.search solution in Clear category for The Longest Palindromic by Serg900vd

import re


def longest_palindromic(a):
    result, index, match = a[0], 'a', True
    regex = '(?P[A-Za-z ])[A-Za-z ](?P=a)'
    while match:
        match = re.search(regex, a)
        if match:
            result = match[0]
            index += 'a'
            regex = f'(?P<{index}>[A-Za-z ]){regex}(?P={index})'
    return result


if __name__ == '__main__':
    print("Example:")
    print(longest_palindromic('abc'))

    # These "asserts" are used for self-checking and not for an auto-testing
    assert longest_palindromic('abc') == 'a'
    assert longest_palindromic('abacada') == 'aba'
    assert longest_palindromic('artrartrt') == 'rtrartr'
    assert longest_palindromic('aaaaa') == 'aaaaa'
    print("Coding complete? Click 'Check' to earn cool rewards!")

March 22, 2022

Comments:

Selindian on Nov. 14, 2022, 2:46 p.m.
For my understanding, solutions are shared to get feedback and give hints and ideas to beginners how to solve things cleverer. Especially when it comes to regex, a few comments for better understanding are usually very helpful. Please don't misunderstand me. I like your solution, but I would have even liked it better with some comments.

Serg900vd on Nov. 16, 2022, 12:50 p.m.
You're right. Regular expressions require some comments. <pre class='brush: python'>def longest_palindromic(a): result, index, match = a[0], 'a', True regex = '(?P<a>[A-Za-z ])[A-Za-z ](?P=a)' # (?P<a> ) -Named Capture Group. # [A-Za-z ] -Match a single character present in the list. # [A-Za-z ] -Match a single character present in the list. # (?P=a)-Matches the same text as most recently matched by # the capturing group named "a". while match: match = re.search(regex, a) if match: result = match[0] index += 'a' regex = f'(?P<{index}>[A-Za-z ]){regex}(?P={index})' # Wrap an additional Named Capture Group for the next loop. # Expressions for step two: # # (?P<aa>[A-Za-z ])(?P<a>[A-Za-z ])[A-Za-z ](?P=a)(?P=aa) return result </pre>

Selindian on Nov. 16, 2022, 1:21 p.m.
Nice. Thank you for the added comments.

Serg900vd on Nov. 16, 2022, 1:31 p.m.
Looking again at my solution, I found the problem. Code falls with aserts like this: <pre class='brush: python'>assert longest_palindromic('aa') == 'aa' assert longest_palindromic('ABBA') == 'ABBA' </pre> After bug fix and refactoring, the solution looks like this: <pre class='brush: python'>def get_next_regex(regex, index): return f'(?P<{index}>[A-Za-z ]){regex}(?P={index})' def longest_palindromic(line): result, index, match = line[0], 'a', True regex_even = '' regex_odd = '[A-Za-z ]' while match: regex_even = get_next_regex(regex_even, index) match = re.search(regex_even, line) if match: result = match[0] regex_odd = get_next_regex(regex_odd, index) match = re.search(regex_odd, line) if match: result = match[0] index += 'a' return result </pre> Thank you for your comment.

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