question on preg_replace()

I'm having a problem figuring out how to prevent preg_replace() from replacing substrings when one element of the pattern array matches a portion of a string that would match a subsequent value in the pattern array. Say I have an two arrays: find => Array ( [0] => 'fred' [1] => 'tom' [2] => 'frederick' ) replace => Array ( [0] => 'person1' [1] => 'person2' [2] => 'person3' ) and a string $string = 'fred knows tom who knows frederick'. So the string contains an instance of each $find value, and I'd like to replace each one with its corresponding $replace value. The problem is that preg_replace() changes 'frederick' to 'person1erick' before even getting to find[2] ('frederick'), which at this point won't find anything to replace, and I'm left with 'person1 knows person2 who knows person1erick' when what I want to end up with is 'person1 knows person2 knows person3.' Also, the actual values in the $find array are not names but strings representing each unique combination of HTML tags in $string (such as '<br>', '<b>', '<br><u>', so that the <u> here would be left over unmatched). It seems to me that my only option is to write a function to sort $find in reverse order by length, so that the longer strings are replaced first. Or am I missing something obvious? Thanks in advance, Andy