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

##</code><code>##

/^(a+)(b+)(c+)$/ && length($1) == length($2) && length($2) == length($3);

##</code><code>##

/^([0-9]+)$(?(?{sqrt($1) == int sqrt($1)})|(*FAIL))/;

##</code><code>##

/^(.*)$(?(?{is_whatever($1)})|(*FAIL))/;