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regexregex-lookaroundsalteryx

How can I simulate a negative lookup in a regular expression

I have the following regular expression that includes a negative look ahead. Unfortunately the tool that I'm using does not support regular expressions. So I'm wondering if its possible to achieve negative look ahead behaviour without actually using one.

Here is my regular expression:

(?<![ABCDEQ]|\[|\]|\w\w\d)(\d+["+-]?)(?!BE|AQ|N)(?:.*)

Here it is working with sample data on Regex101.com:

see expression on regex101.com

I'm using a tool called Alteryx. The documentation indicates that it uses Perl, however, for whatever reason the look ahead does not work.

Solution

  • Alteryx appears to use the Boost library for its regex support, and the Boost documentation says lookbehind expressions must have a fixed length. It's more restrictive than PHP (PCRE), which allows you to use alternation in a lookbehind, as long as each branch is fixed-length. But that's easy enough to get around: just use multiple lookbehinds:

    (?<![ABCDEQ])(?<!\[)(?<!\])(?<!\w\w\d)(\d+["+-]?)(?!BE|AQ|N)(?:.*)

    That regex works for me in a Boost-powered regex tester, where yours doesn't. I would compress it a little more by putting square brackets inside the character set:

    (?<![][ABCDEQ])(?<!\w\w\d)(\d+["+-]?)(?!BE|AQ|N)(?:.*)

    The right bracket is treated as a literal when it's the first character listed, and the left bracket is never special (though some other flavors have different rules).

    Here's the updated demo.