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Don't use a regexp. It isn't clear what you want to happen if there isn't a middle character if the string has a odd integer for its length. Try using some type of substring approach such as this:

str  xxx"
str[0..str.length/2-1] + 
str[str.length/2,1] * 4 + 
str[str.length/2+1..str.length]
"xxxxxx"


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