How could we match the language 0{n}1{n}?

If you already know that $_ contains '0' and '1' chars only: 

tr/0// == tr/1// and index $_, '10' < 0

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else: 
my $hl = (1+length) >> 1;
tr/0// == $hl and tr/1// == $hl and index $_, '10' < 0

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or just 
/^(0*)(1*)$/ and length $1 == length $2

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what if I only wanted the number of 0s and the number of 1s to be equal? 

tr/0// == tr/1//

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In reply to Re: A (non) reg-ex question by salva
in thread A (non) reg-ex question by Ido

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