search and replace when needed

[dcole.ath.cx]

How would you go about searching and replacing something like this:I want to see if there is 123 in a file,if it is, I want to replace it with 123456but if there is 123456 I don't want it to add another 456 to it! (I don't want 123456456)What sould I do?------How about searching for 123, then taking the part I found and adding the parts around it , then search for 123456 and if that's there move on! Other wise replace 123 with 123456.so when it finds 123 it will take 123 and 3 charactors after it, then search for 123456Does this sound good?