I am working on some code code that boils down to:

'g'=~/g/;
my $regex = 'm{}g';  # user input
my $text = 'x';      # user input
eval qq{
    use re 'debug';
    print "<\$&>\\n" while \$text =~ $regex;
};

[download]

Which only produces this output:

Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0 
Freeing REx: ""

[download]

As per perlop:

The empty pattern //

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the g and c flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match).

So in other words, it's silently (!) matching $text against /g/. Is there some way to reset the state of the regex engine so that the empty pattern always acts as a genuine empty pattern? it acts as if no other regex has been executed before $regex? I think that'd make more sense to the users instead of the current confusing behavior. (I think I'll display a message to the user about the empty pattern in any case.)

Update: Updated explanation in final paragraph.

In reply to Reset meaning of empty pattern? by haukex

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