Can We Achieve True Variable-Length Lookbehind in Regular Expressions?

Variable-Length Lookbehind-Assertion Alternatives for Regular Expressions

Variable-length lookbehind assertions in regular expressions, denoted by (?

Python with regex Module

The Python regex module offers support for variable-length lookbehind assertions.

<code class="python">import regex

m = regex.search('(?<!foo.*)bar', 'f00bar')
print(m.group())  # Output: bar</code>

K Token

Modern regular expressions introduce the K token, which ensures that matching characters before it are not included in the match.

s/unchanged-part\Kchanged-part/new-part/x

Limitations

While K provides some lookbehind functionality, it cannot remove characters up to a specific point or be used multiple times in an expression.

Enhanced Regular Expression Implementations

Besides regex for Python, other enhanced regular expression implementations may exist for Perl, Ruby, JavaScript, and PHP. Investigating these alternatives may provide additional options for handling variable-length lookbehind assertions.

Questions

• Can K's effect be extended to a specific point in the match?
• Are there any enhanced regular expression implementations for Perl, Ruby, JavaScript, or PHP that support variable-length lookbehind assertions?

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