The first one is true if, and only if, there is a colon in the string (assuming no newlines in the string). Hence, the second alternative will never be selected. If that would match, the first alternative would have matched already.

I'd do something like: 

if (/$_[0] =~ /^([^:]+):([^|]+)(?:\|(.*))?$/ ||
     $_[0] =~ /^([^|)+)\|(.*)$/) {
    @key{qw /name definition altdefinition/} = ($1, $2, defined $3 ? $
+3 : "")
}

[download]
It would even be simpler if you used one kind of delimiter, say the pipe. Then you could do: 
if ($_[0] =~ /\|/) {
  @key{qw /name definition altdefinition/} = split /\|/, $_[0];
  $key{altdefinition} = "" unless defined $key{altdefinition};
}

[download]
I certainly wouldn't call by sub 'split'.

In reply to Re: Getting regular expression right by Anonymous Monk
in thread Getting regular expression right by mellin

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