Regexp

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. It will appear in the minibuffer as ^J.

See Also

Re-builder build regexp interactively in buffer