1 . ^([0-9A-Za-z]{5})+$
vs
2 . ^[a-zA-Z0-9]{5}+$
My intention is to match any string of length n such that n is a multiple of 5.
Check here : https://regex101.com/r/sS6rW8/1.
Please elaborate why case 1 matches the string whereas case 2 doesnot.
Because {n}+ doesn't mean what you think it does. In PCRE syntax, this turns {n} into a possessive quantifier. In other words, a{5}+ is the same as (?>a{5}). It's like the second + in the expression a++, which is the same as using an atomic group (?>a+).
This has no use with a fixed-length {n} but is more meaningful when used with {min,max}. So, a{2,5}+ is equivalent to (?>a{2,5}).
As a simple example, consider these patterns:
^(a{1,2})(ab) will match aab -> $1 is "a", $2 is "ab"
^(a{1,2}+)(ab) won't match aab -> $1 consumes "aa" possessively and $2 can't match
In ^([0-9A-Za-z]{5})+$ you're saying any number or letter 5 characters long 1 or more times. The + is on the entire group (whatever's inside the parentheses) and the {5} is on the [0-9A-Za-z]
Your second example has a no backtrack clause {5}+, which is different than (stuff{5})+
