Here is one way to do it: s/(bad)/$1 & (' ' x length $1) | 'SAD'/ie; This works because the ASCII code for space (0x20) is exactly the bit that distinguishes between ASCII upper and lower case letters.

$1 & (' ' x length $1) returns a string containing nulls and spaces. Each uppercase character in $1 produces a null in the string, and each lowercase character produces a space.

Or-ing that with 'SAD' turns on the "lowercase bit" for each letter in 'SAD' that lines up with a space in our string. The characters that line up with nulls are unchanged.

So, effectively, this copies all the lowercase bits from $1 to 'SAD'.

This is hard for me to explain, please let me know if it's not clear.