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Here is a simple program to search a text file  for a user specified string. 

The substring search function, Pos, is used to perform the basic search of each line of the input text file.  

If the "Case sensitive" checkbox is checked, the search is performed "as is".  If "Case Sensitive" is not checked, then both the input line and the input string are converted to upper case before the "Pos" search is performed.

If "Whole words only" is checked a call is made to function IsWord   which returns true two conditions are met:  

• The position before the first letter of the search string is a delimiter or 0 (i.e. the string begins at the beginning of the line). 
• The position after the last letter of the search string is a delimiter or past the end of the line (i.e. the string ends at the end of the line). 

One more check box, "Show hits only" ,  allows user  to display only the lines with matches or  to display the entire input file with matched strings  highlighted.   In order to highlight the matches strings, a TRichEdit is used.  The matched strings are selected using Selstart and SeLength properties and setting SelAttributes  to make the font style bold.  The same technique could also apply color or other style properties to matches.   One trick is used  to highlight multiple occurrences of the search string in the displayed input line - each occurrence is replaced by  non-displayable characters in the working copy of the line before the next search is performed.

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