regex101: Palindromes

Definition Group(?(DEFINE) (?<palindrome> # Recursive alternative first to match recursive palindromes. # Invert alternatives order to match nested palindromes individually # and (drastically) reduce backtracking. (?<l1>\p{L})\p{M}* [\s\p{P}]* (?&palindrome) [\s\p{P}]* \k<l1>\p{M}* | (?<l2>\p{L})\p{M}* [\s\p{P}]* \k<l2>\p{M}* | \p{L}\p{M}* ) )

Group palindrome(?<palindrome> # Recursive alternative first to match recursive palindromes. # Invert alternatives order to match nested palindromes individually # and (drastically) reduce backtracking. (?<l1>\p{L})\p{M}* [\s\p{P}]* (?&palindrome) [\s\p{P}]* \k<l1>\p{M}* | (?<l2>\p{L})\p{M}* [\s\p{P}]* \k<l2>\p{M}* | \p{L}\p{M}* )

1st Alternative # Recursive alternative first to match recursive palindromes. # Invert alternatives order to match nested palindromes individually # and (drastically) reduce backtracking. (?<l1>\p{L})\p{M}* [\s\p{P}]* (?&palindrome) [\s\p{P}]* \k<l1>\p{M}*

Comment: Recursive alternative first to match recursive palindromes.

Comment: Invert alternatives order to match nested palindromes individually

Comment: and (drastically) reduce backtracking.

Group l1(?<l1>\p{L})

\p{L}any Unicode letter

\p{M}*a combining mark, repeated any number of times

Character Class[\s\p{P}]*

*matches any number of characters from the set below:

\sany whitespace character

\p{P}a punctuation character

(?&palindrome)calls group palindrome as a subroutine

Character Class[\s\p{P}]*

*matches any number of characters from the set below:

\sany whitespace character

\p{P}a punctuation character

\k<l1>the text saved by group l1

\p{M}*a combining mark, repeated any number of times

2nd Alternative (?<l2>\p{L})\p{M}* [\s\p{P}]* \k<l2>\p{M}*

Group l2(?<l2>\p{L})

\p{L}any Unicode letter

\p{M}*a combining mark, repeated any number of times

Character Class[\s\p{P}]*

*matches any number of characters from the set below:

\sany whitespace character

\p{P}a punctuation character

\k<l2>the text saved by group l2

\p{M}*a combining mark, repeated any number of times

3rd Alternative \p{L}\p{M}*

\p{L}any Unicode letter

\p{M}*a combining mark, repeated any number of times

Positive Lookbehind(?<=[\s\p{P}]|^)

1st Alternative[\s\p{P}]

Character Class[\s\p{P}]

\sany whitespace character

\p{P}a punctuation character

2nd Alternative^

^start of string

(?&palindrome)calls group palindrome as a subroutine

Condition(?(?=\s*\p{P}) (?:\s*\p{P})+ | (?=\s|$))

Uses the first branch if the lookaround matches

Positive Lookahead(?=\s*\p{P})

\s*any whitespace character, repeated any number of times

\p{P}a punctuation character

If the condition is true, match this branch: (?:\s*\p{P})+

Non-Capturing Group(?:\s*\p{P})+

+repeats the group contents below at least once:

\s*any whitespace character, repeated any number of times

\p{P}a punctuation character

Otherwise, match this branch: (?=\s|$)

Positive Lookahead(?=\s|$)

1st Alternative\s

\sany whitespace character

2nd Alternative$

$end of string, or before its final line terminator

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

x modifier: extended. Ignores unescaped whitespace and comments outside character classes

25 tokens

Regular Expression

(?(DEFINE)
  (?<palindrome>
    # Recursive alternative first to match recursive palindromes.
    # Invert alternatives order to match nested palindromes individually
    # and (drastically) reduce backtracking.
    (?<l1>\p{L})\p{M}* [\s\p{P}]* (?&palindrome) [\s\p{P}]* \k<l1>\p{M}*
    | (?<l2>\p{L})\p{M}* [\s\p{P}]* \k<l2>\p{M}*
    | \p{L}\p{M}*
  )
)

(?<=[\s\p{P}]|^) (?&palindrome) (?(?=\s*\p{P}) (?:\s*\p{P})+ | (?=\s|$))

Unit Tests

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