This article is intended as a simple introduction to regular expression. I will show it in search & replace feature of a text editor. I will use Kate here, but it is applicable to another text editors available such as Gedit. I will show you some short examples to make it easy. It is not the best, but I hope it gives you global overview of regex in Linux text editors.
This is the text being edited. This is basically just a table of contents I create in LibreOffice. It contains many lines, every line with beginning numbers -> subtitle -> dots -> then page number. We will "play" with them. I will use this text as example. You may download this text file in http://pastebin.com/Z6SpwWyn.
Enable Regex Searching
In Kate, press Ctrl+R to open Search & Replace facility. Then select Regular expression from Mode selection.
Find The Page Numbers
Because every page number contains number, we use [0-9] regex here. Because every page number may contain one or two or three number, we use * regex here. Because the page number is always in the end of line, we use $ regex here. So, complete regex will be \s[0-9]*$. See picture below.
Find The Dots
The dots on the center of every line have no same exact number. There is many dots, and every line has different number of it. But at least a line has three continuous dots so we use \.\.\. regex and we add * regex in the end. We use three dots to distinguish it with dot in the beginning of every line. A single dot ( . ) has special meaning in regex, so we don't use it. We use instead a single slash single dot ( \. ) as escape character, as a normal dot matching with real dot in text. See picture below.
Find The Beginning Numbers
Our beginning numbers in each line has format X.X. (notice the trailing dot) or X.XX. (notice the trailing dot too). So, the regex will be [0-9] for first one single number. Then, we add \. regex as replacement of dot (real dot in text) in the middle. Then, we add again [0-9][0-9] but we add ? regex in the middle of both so it is [0-9]?[0-9]. Regex question mark ( ? ) makes previous character optional, so it makes 1 (single number) or 11 (double number) match. We add again \. in the end so it will match real dot in text you've noticed above. Finally, we add ^ regex to match this regex in the exactly beginning of every line. So the final regex will be ^[0-9]\.[0-9]?[0-9]\.. See picture below.