This regular expression explain that.
/^ => The starting of the variable
^\d => The digit should not come
{2,4} => previous char should appear min = 2 , max = 4 times here previous char is any of the alphabets
< => the char '<'
( => it is grouping
^>+ => other then '>' more then one char
) => group is close
> => the char '<'
\d? => one or more digit
\1 => matched group is again
$/ => The End
Here the matched string variable is $1. it has assigned when it match.
so it defined then it prints the matched word