Hello, @blake-g.-barrington, @peterjones, @coises and All, Here is my version to get duplicate lines, separated by any number of lines ( whatever they are : true empty lines, blank lines or lines with text ), even zero line : MARK (?-si)^(.+)(?=(.*\R)+\1$) As said by @Peterjones, and after tests, this kind of regex get into trouble when the bunch of lines, between two duplicate lines, exceeds 15,000 empty lines about ! Now, Let’s suppose you create this text file : first 0002 0003 0004 0005 .... .... .... 9997 9998 9999 10000 first Then the (?-si)^(.+)(?=(.*\R)+\1$) regex returns : Correct result until the number of the in-between lines does not exceed 1,600, with the message 1 match in entire file Correct result until the number of the in-between lines does not exceed 12,000, although the error message Find Invalid Regular Expression ( So F.I.R.E. !! ) occurs ! NO result at all if the number of lines is over 12,000 lines, with, of course, the error message Find Invalid Regular Expression Best Regards, guy038

A small example of what is already working: [image: 1788632411234-cf4e0aa7-d7a1-4c9c-a14f-db8fb19ae2a3-image.jpeg] This demonstrates just several functions; the other functions from USAGE_AND_SYNTAX work as well. The next step is to: Create a syntax file to apply “function color” to built-in functions; Apply this syntax file automatically when Smart Math is enabled for the current file/document. The syntax file can be created by means of user-defined language. But how to apply this user-defined language automatically when Smart Math is enabled for the current file/document?