Regexp

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

See Also

Re-builder build regexp interactively in buffer