My interpretation of the behaviour is that you're running afoul of ß (sz-ligature) matching SS when doing a case-insensitive match. This means that Perl will try to match s "inside" of ß, which then results in a variable-length look-behind pattern.

This theory easily explains the first behaviour, as Perl compiles

/(?<!ss)/i

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to

/(?<!ß|ss)/i

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This creates a variable-length lookbehind, which is what Perl does not like.

For your next examples, each "s" might start "inside" of an (expanded) ß:

    ßtabc
   SSTABC
    STABC

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I don't know why your third example compiles when the others do not, but I blame this on the inconsistency of ß matching ss. My suggestion would be to try to avoid /i and instead enumerate the alternatives if possible. That would mean to expand at least the critical words starting with s to character classes matching the upper- and lower-case variant:

/(?<!s)tart/i ;

/(?<![Ss])[Tt][Aa][Rr][Tt]/ ;

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Maybe you can fudge things by constructing your master regular expression from even more parts:

my $not_s= qr/(?<![Ss])/; # no explicit /i here, will never match insi
+de ß
my $rest= qr/abc/i;
my $pattern= "$not_s$rest";

...

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I don't know why it doesn't happen for statically compiled regular expressions. Maybe this is the real bug here.

In reply to Re: qr/STRING/ fails with certain lookbehind expressions by Corion
in thread qr/STRING/ fails with certain lookbehind expressions by wiewa

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