display dialog( my searchAndReplace("my mommy", "my", "your") )

on searchAndReplace( txt, srch, rpl )
    set oldtid to AppleScript's text item delimiters
    set AppleScript's text item delimiters to {srch}
    set temp to every text item of txt
    set AppleScript's text item delimiters to {rpl}
    set temp to (temp as string)
    set AppleScript's text item delimiters to oldtid
    return temp
end searchAndReplace