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Text Pattern Matching in Emacs (emacs regex tutorial)

Xah Lee, 2007-08, 2009-08-20, 2011-01-13

Emacs's regex is not based on Perl or Python's, but is very similar. 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]+	sequence of digits
[A-Za-z]+	sequence of letters
[-A-Za-z0-9]+	sequence of letter, digit, hyphen
[_A-Za-z0-9]+	sequence of letter, digit, underscore
[-_A-Za-z0-9]+	sequence of letter, digit, hyphen, underscore
[[:blank:]]+	sequence of tabs and spaces
[[:upper:]]+	sequence of cap letters
[[:lower:]]+	sequence of lowercase letters
"\([^"]+?\)"	capture text between double quotes (non-greedy)
“\([^”]+?\)”	capture text between curly double quotes (non-greedy; unicode char)
(\([^)]+?\))	capture text between parenthesis (non-greedy)
+	means match previous pattern 1 or more times
*	means match previous pattern 0 or more times
?	means match previous pattern 0 or 1 time

Differences from Perl's Regex

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

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)
When using interactive commands for find ＆ replace, emacs won't understand \t, \n. To enter a literal Tab, press 【Ctrl+q Tab】. To match a new line, press 【Ctrl+q Ctrl+j】. (For explanation, see: Emacs's Key Notations Explained (/r, ^M, C-m, RET, <return>, M-, meta)) In elisp string, you can use \t, \n and no need to double the backslash.
When doing find ＆ replace in a buffer, the line ending char is always represented by \n (Line Feed; ASCII 10). You do not need to worry whether the file has unix style line ending or Windows or Mac. Also, you should NOT change a file's eol style by doing find ＆ replace. (See: Emacs Line Return and Windows, Unix, Mac, All That ^M ^J ^L.)

Test Your Regex

Emacs has a interactive regex mode. It show matches as you type. To go into the mode, call “regexp-builder”.

Alternatively, you can call “query-replace-regexp” to test your pattern.

Test Regex for Elisp

To test regex in your elisp code, you can open a empty file and place the regex function at top and the text you want to match below it, like this:

(search-forward-regexp "yourRegex")

whatever text here

Then, put your cursor to the right of the closing parenthesis, then call “eval-last-sexp”. If your regex matches, it'll move cursor to the last char of the matched text. If you get a lisp error saying search failed, then your regex didn't match. If you get a lisp syntax error, then you probably screwed up on the backslashs.

Double Backslash in Lisp Code

In a lisp regex function that takes a regex string (e.g. “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:

\(\[[a-z]\]\)