If you search for "time" using a search utility, how do you highlight the word "time" throughout the results, without also highlighting "sometime" or "timeframe" or other variations? The trick is to use Regular Expressions, or pattern matching.
This assumes someone submitted a form with a keyword to be searched. These tags work on the results before they are displayed.
What does the \2 \3 and \4 mean? These are numbers to references pieces of the input to a regular expression function like REReplaceNoCase(). For example, see the following:
(\1 (\2) (\3) (\4 (\5) ) )
Look at the placement of parentheses and numbers, \1 starts at the outermost pair of parentheses, and working your way in increases every number. If they are on the same level of the hierarchy of parentheses, it goes from left to right in the expression.