You have a large number of files on which you want to do a regex search and replace. You want the replacement string to be an arithmetic expression of a part of the regex match. To give a concrete example, say you have a few files which contain times specified in a particular format and you need to do a format conversion. For instance, convert “PT30M” to “1800” (30 * 60 seconds)
You can use Emacs regex search and replace with simple arithmetic expressions. The key is to prefix the arithmetic expression with \, which tells Emacs that it’s a LISP expression instead of a string.
For instance to do the aforementioned conversion, type M-x replace-regexp. Then type PT([0-9]+)M RET \,( 60 #1)*
In the above expression, you need to indicate that the regex group is numeric by specifying #1 instead of just \1. (Otherwise you will get a type error)
If you have a large number of files on which you want to do the above search and replace, you can use dired. Type M-x dired. Then mark the desired files typing m. Then type Q. This will do a* query-replace-regexp* on each marked file.