An effective variable length look-behind is available in Perl 5.10 with the  \K special escape. The following compiles
    my $rx = qr/(?:^| )\K($phrases_re)(?= |$)/;
but whether it serves the OPer's true needs is another question.

In reply to Re^3: phrase match by AnomalousMonk
in thread phrase match by newbio

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