Regexp
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Origin
Regexp is a portmanteau of the words regular and expression. It is the Emacs abbreviation for "regular expression". Many other computer languages and software use the abbreviation regex (no trailing p) instead.
Wikipedia article about regular expressions
Syntax
The following characters are special : . * + ? ^ $ \ [
Between brackets [], the following are special : ] - ^
Many characters are special when they follow a backslash – see below.
. any character (but newline)
* previous character or group, repeated 0 or more time
+ previous character or group, repeated 1 or more time
? previous character or group, repeated 0 or 1 time
^ start of line
$ end of line
[...] any character between brackets
[^..] any character not in the brackets
[a-z] any character between a and z
\ prevents interpretation of following special char
\| or
\w word constituent
\b word boundary
\sc character with c syntax (e.g. \s- for whitespace char)
\( \) start\end of group
\< \> start\end of word
\` \' start\end of buffer
\1 string matched by the first group
\n string matched by the nth group
\{3\} previous character or group, repeated 3 times
\{3,\} previous character or group, repeated 3 or more times
\{3,6\} previous character or group, repeated 3 to 6 times
.?, +?, and ?? are non-greedy versions of ., +, and ? \W, \B, and \Sc match any character that does not match \w, \b, and \sc
Character category
\ca ascii character \Ca non-ascii character (newline included) \cl latin character \cg greek character
POSIX character classes
[:digit:] a digit, same as [0-9] [:upper:] a letter in uppercase [:space:] a whitespace character, as defined by the syntax table [:xdigit:] an hexadecimal digit [:cntrl:] a control character [:ascii:] an ascii character
Syntax classes
\s- whitespace character \s/ character quote character \sw word constituent \s$ paired delimiter \s_ symbol constituent \s' expression prefix \s. punctuation character \s< comment starter \s( open delimiter character \s> comment ender \s) close delimiter character \s! generic comment delimiter \s" string quote character \s| generic string delimiter \s\ escape character
Regexp Examples
[-+[:digit:]] digit or + or - sign \(\+\|-\)?[0-9]+\(\.[0-9]+\)? decimal number (-2 or 1.5 but not .2 or 1.) \(\w+\) +\1\> two consecutive, identical words \<upper:\w* word starting with an uppercase letter +$ trailing whitespaces (note the starting SPC) \w\{20,\} word with 20 letters or more \w+phony\> word ending by phony \(19\|20\)[0-9]\{2\} year 1900-2099 ^.\{6,\} at least 6 symbols ^[a-zA-Z0-9_]\{3,16\}$ decent string for a user name <tag[^> C-q C-j ]*>\(.*?\)</tag> html tag
Emacs Commands that Use Regular Expressions
C-M-s incremental forward search matching regexp C-M-r incremental backward search matching regexp replace-regexp replace string matching regexp query-replace-regexp same, but query before each replacement align-regexp align, using strings matching regexp as delimiters highlight-regexp highlight strings matching regexp occur show lines containing a match multi-occur show lines in all buffers containing a match how-many count the number of strings matching regexp keep-lines delete all lines except those containing matches flush-lines delete lines containing matches grep call unix grep command and put result in a buffer lgrep user-friendly interface to the grep command rgrep recursive grep dired-do-copy-regexp copy files with names matching regexp dired-do-rename-regexp rename files matching regexp find-grep-dired display files containing matches for regexp with Dired
Tips and Tricks
To enter a newline character in a regexp, use the two keystroke sequence C-q C-j.
See Also
Re-builder build regexp interactively in buffer