Text Pattern Matching in Emacs

Xah Lee, 2007-08.

Emacs's regex is not based on Perl or Python's, but it is basically the same, except in emacs regex, the parenthesis characters “()” are literal. If you want to capture a pattern, you need to escape the paren like this: “\(myPattern\)”.

Common Patterns

Here are some common patterns:

Pattern         Matches
-----------------------------------
.               any single character
\.              one period

[0-9]+          digit sequence
[A-Za-z]+       sequence of letters
[_A-Za-z0-9]+   sequence of alphanumeric char and underscore
[-A-Za-z0-9]+   sequence of alphanumeric char and hyphen
[[:blank:]]+    sequence of tabs and spaces

"\([^"]+\)"     capture text between double quotes 

+               means match previous pattern 1 or more times
*               means match previous pattern 0 or more times
?               means match previous pattern 0 or 1 time

The above patterns will cover the vast majority of regex uses.

Differences from Perl's Regex

If you are familiar with Perl's regex, here are some practical major difference.

• Parenthesis is literal in emacs's regex. Use “\(pattern\)” to capture instead.
• Do not use “[A-z]”, as its meaning is currently ambiguous. Use “[A-Za-z]”.
• Character classes like “/d”, “/w”, “/s” does not work. Use “[[:digit:]]”, “[[:word:]]”, “[[:space:]]” instead. For example, perl's “/d+” for one or more digits is emacs's “[[:digit:]]+”. (Yes, you need double brackets)
• Meta chars like “\t”, “\r”, “\n” does not work in interactive commands. To enter a literal tab, press “Ctrl+q TAB”. To enter a carriage return (ascii 13), press “Ctrl+q RET”. To enter a line feed (unix EOL; ascii 10), press “Ctrl+q Ctrl+j”. (For explanation, see: The Confusion of Emacs's Keystroke Representation)
• In elisp code, meta chars like “\t”, “\r”, “\n” does work. (and, no need to double the backslash)

Testing Your Regex

Emacs has a interactive regex mode, so that you can type a pattern and see immediately what it matches, as you type out the regex. To go into the mode, type “Alt+x regexp-builder”.

Alternatively, you can type “Alt+x query-replace-regexp” to test your pattern. Its keyboard shortcut is “Ctrl+Alt+%”.

To test regex in lisp code, you can open a empty file and place this code “(search-forward-regexp "yourRegex")” then place the text you want to match below it. Then, move your cursor right next of the closing parenthesis, then type “Ctrl+x Ctrl+e” (or Alt+eval-last-sexp). If your regex matches, it'll move to the last char of the matched text. If you get a lisp error saying search-failed, then you know your regex didn't match. If you get a lisp syntax error, then you know you probably screwed up on the backslashs.

Double Backslash in Lisp Code

In a lisp regex function that takes a regex string (Example: “search-forward-regexp”), you will need to use double backslash. This is because, in elisp string, a backslash needs to be prefixed with a backslash, then, this interpreted string is passed to emacs's regex engine.

For example, suppose you have this text “Sin[x] + Sin[y]”, and you need to capture the x or y. You can use “(search-backward-regexp "\\(\\[[a-z]\\]\\)")”. The regex engine really just got “\(\[[a-z]\]\)”.

If you want to match “\n”, you don't need double backslash because “\n” already stand for line return inside a string.

Reference: (info "(emacs)Regexps").

Reference: Elisp Manual: Regular-Expressions.

Related essays:

• Emacs and Unix Tips
• Interactively Find and Replace String Patterns on Multiple Files
• Rename Files by a Text Pattern

Page created: 2007-08.
© 2007 by Xah Lee.