repeat loop and formatting question

[Mark Swindell]

What would be the most efficient structure and syntax to achieve the  
following?:

A text field containing several paragraphs. I want to run through the  
field, finding and formatting certain strings, all occurrences of a  
list of certain letter combinations:

example, all occurrences of
"ck" = bold
"ch" = bold
"oa" = underline
"oo" = underline
  etc.

The repeat loops I have tried run slowly and there are too many  
unnecessary passes (it seems).  I know there must be a more succinct,  
efficient statement that should work.

Any suggestions would be appreciated.

Thanks,
Mark