What is Regular Expression? A regular expression is a special text string for describing a certain amount of text. Generally, A regular expression contains a few special characters and ordinary characters. If you have used Linux/Windows systems, you will probably familiar with wildcard notations such as *.txt to find all text files in a folder. That’s a simple regular expression example (Thanks for netcasper’s correcting . The regex equivalent to “.txt” is .*.txt). Nowadays, Most of the text editors support regular expression such as vi, emacs and Ultra Editor(not a free software). Today, I would like introduce some basic knowledge about regular expression and use it in Emacs to improve the efficiency of your coding work.