http://www.perlmonks.org?node_id=803019


in reply to Re^3: check for square-number with a regex
in thread check for square-number with a regex

I think you're aware of this solution for anbncn but I'll post it anyway(it uses ??{} blocks and ?{} and is a Perl regex). 
$r = qr/^(a+b)(?{ $a = -1+length $^N; })(??{ $b=$a-1;"(b{$b})" })(??{ 
+"(c{$a})" })/x;

sub test {
        print
        $_[0] =~ $r
        ? "matched\n"
        : "not matched\n";
}

test "aaabbbccc";
test "aaaabbbccc";
test "aaaabbbbccc";
test "aaaabbbbcccc";
test "aabbbbcccc";
test "abc";
test "aabbcc";

[download]
Result on Perl v5.8.8: 
matched
not matched
not matched
matched
not matched
matched
matched

[download]

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Re^5: check for square-number with a regex
by JavaFan (Canon) on Oct 24, 2009 at 11:54 UTC
    I (and others in this thread) explicitely mentioned no using (?{}) and (??{}). Allowing those means you have the full power of Perl available. It would mean that anything you can do in Perl, you can to with a "regular expression", and the question "can I do foo with a regular expression" becomes a non-question.

    And I would have used (?{}) differently anyway. Something like: 

    /^(a+)(b+)(c+)$(?(?{length($1) == length($2) && length($2) == length($
+3)})|(*FAIL))/;

    [download]
    But that's just a fancy way of writing: 
    /^(a+)(b+)(c+)$/ && length($1) == length($2) && length($2) == length($
+3);

    [download]
    And you can solve the "match a square" with: 
    /^([0-9]+)$(?(?{sqrt($1) == int sqrt($1)})|(*FAIL))/;

    [download]
    Or in general, match "whatever" by: 
    /^(.*)$(?(?{is_whatever($1)})|(*FAIL))/;

    [download]
    after writing the appropriate "is_whatever" function.

    But that means the question no longer is "how can I do foo with a regexp", but "how can I do foo in Perl".

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