var str = "I am a string";

# Substitute something mached by a regex
str.sub!(/ a /, ' another ');   # "I am a string" => "I am another string"

# Remove something matched by a regex
str -= / \Kanother /i;          # "I am another string" => "I am string"

# Global subtitution with a block
str = str.gsub(/(\w+)/, {|s1| 'x' * s1.len});  # globaly replace any word with 'xxx'

say str;     # prints: 'x xx xxxxxx'