A regular expression (abbreviated “regexp” or sometimes just “re”) is a search-string with wildcards – and more. It is a pattern that is matched against the text to be searched. See Regexps. Examples:
"alex"A plain string is a regular expression that matches the string exactly. The above regular expression matches “alex”.
"alexa?"Some characters have special meanings in a regular expression. The question mark, for example, says that the preceding expression (the character “a” in this case) may or may not be present. The above regular expression matches “alex” or “alexa”. S Regexps are important to Emacs users in many ways, including these:
‘C-M-s’ (command isearch-forward-regexp).Here is the syntax used by Emacs for regular expressions. Any character matches itself, except for the list below.
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 (faulty rendering: backslash + less-than and backslash + greater-than) \_< \_> start/end of symbol \` \' start/end of buffer/string \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 \= match succeeds if it is located at point
*?, +?, and ?? are non-greedy versions of *, +, and ? – see NonGreedyRegexp. Also, \W, \B, and \Sc match any character that does not match \w, \b, and \sc.
Characters are organized by category. Use C-u C-x = to display the category of the character under the cursor.
\ca ascii character \Ca non-ascii character (newline included) \cl latin character \cg greek character
Here are some syntax classes, also known as character classes, that can be used between brackets, e.g. [[:upper:]\|[:digit:]\.].
[:digit:] a digit, same as [0-9] [:alpha:] a letter (an alphabetic character) [:alnum:] a letter or a digit (an alphanumeric character) [:upper:] a letter in uppercase [:lower:] a letter in lowercase [:graph:] a visible character [:print:] a visible character plus the space character [:space:] a whitespace character, as defined by the syntax table, but typically [ \t\r\n\v\f], which includes the newline character [:blank:] a space or tab character [: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
You can see the current syntax table by typing C-h s. The syntax table depends on the current mode. As expected, letters a..z are listed as word constituents in text-mode. Other word constituents in this mode include A..Z, 0..9, $, %, currency units, accented letters, kanjis. See EmacsSyntaxTable for details.
M-s SPC while searching or replacing to toggle between this behavior and treating spaces as literal spaces. Or put the following in your InitFile to override this behaviour.(setq search-whitespace-regexp nil)
[^ … ] matches all characters not in the list, even newlines. Put a newline in the list if you want it not to be matched. You can enter a newline character using ‘C-o’, ‘C-q C-j’, ‘C-q 012 RET’, or as “\n” within an Emacs Lisp string. Note also that \s- matches whitespace, sometimes including newlines, but is mode-dependent.john by harry below. Case conversion can be toggled on/off by typing ‘M-c’ in the minibuffer during search. You can also set the variable case-fold-search to nil to disable case conversion; see CaseFoldSearch for more details. In the following example, only the last line would then be replaced. John => Harry
JOHN => HARRY
john => harry
\n for newline, \t for tab, \b for backspace, \u3501 for character with unicode value 3501, and so on. Backslashes must be entered as \\. Here are two ways to replace the decimal point by a comma (e.g. 1.5 -> 1,5), first by an interactive command, second by executing Lisp code (type C-x C-e after the expression to get it executed).M-x replace-regexp RET \([0-9]+\)\. RET \1, RET (while (re-search-forward "\\([0-9]+\\)\\." nil t) (replace-match "\\1,"))
\d for any digit is not supported, use [0-9] or [:digit:] instead.[-+[: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
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
Note that list-matching-lines is an alias for occur and delete-matching-lines is an alias for flush-lines. The command highlight-regexp is bound to C-x w h. Also query-replace-regexp is bound by default to C-M-%, although some people prefer using an alias, like M-x qrr. Put the following in your InitFile to create such alias.
(defalias 'qrr 'query-replace-regexp)
See also: IncrementalSearch, ReplaceRegexp, AlignCommands, OccurBuffer, DiredPower
‘re-builder’ constructs a regular expression. You enter the regexp in a small window at the bottom of the frame. The first 200 matches in the buffer are highlighted, so you can see if the regexp does what you want. Use Lisp syntax, which means doubling backslashes and using \\\\ to match a literal backslash.‘rx’ provides user-friendly syntax for regular expressions. For example, (rx (one-or-more blank) line-end) returns the regexp string "\\(?:[[:blank:]]+$\\)". See rx.‘rx’.‘C-M-s’) is a great way to learn about regexps – see Regexp Searches. Change your regexp on the fly and see immediately what difference the change makes.C-M-s SPC+$M-x highlight-regexp RET SPC+$ RET RETM-x replace-regexp RET SPC+$ RET RET (same as ‘M-x delete-trailing-whitespace’)C-M-s \s( C-M-s \(\<\w+\>\)\s-+\1M-x how-many RET \< RETM-: (setq case-fold-search nil) RET then M-x align-regexp RET \<[[:upper:]][[:lower:]] RETfoo by bar (won’t replace fool by barl): M-x replace-regexp RET \<foo\> RET barM-x replace-regexp RET ^\(\W*\w+\W+\w+\).* RET \1 RET;;: M-x flush-lines RET ^;; RET; on each line: M-x replace-regexp RET \([^;]*\);.* RET \1 RETM-x keep-lines RET \w+\(\.\w+\)?@\(\w\|\.\)+ RETM-x replace-regexp RET ^C-q C-j\{2,\} RET C-q C-j RETM-x replace-regexp RET [^[:upper:]]+ RET C-o RETChapter or Section: M-x occur RET ^\(Chapter\|Section\) RETM-x occur RET ^.\{81,\} RETIcicles provides these interactive ways to learn about regexps:
C-`’ (‘icicle-search’) shows you regexp matches, as does ‘C-M-s’, but it can also show you (that is, highlight) regexp subgroup matches. Showing matched subgroups is very helpful for learning. There are two ways that you can use this Icicles feature:‘defun’: (defun [^(]*\(([^(]*)\), that is, defun, followed by non-`(’ character(s), followed by `(’, possibly followed by non-`)’ character(s), followed by `)’.(\([-a-z*]+\) *\((\(([-a-z]+ *\([^)]*\))\))\).* is matched:C-`’ also helps you learn by letting you use two simple regexps (search within a search) as an alternative to coming up with a single, complex regexp to do the same job. And, as with incremental search, you can change the second regexp on the fly to see immediately what difference the change makes. See Icicles - Search Commands, Overview‘S-TAB’ during minibuffer input shows you all matches for your input string, which can be a regexp. So, just type a regexp whenever the minibuffer is active for completion and hit ‘S-TAB’ to see what the regexp matches. Try this with command input (‘M-x’), buffer switching (‘C-x b’), file visiting (‘C-x f’), help (‘C-h f’, ‘C-h v’), and so on. Almost any time you type input in the minibuffer, you can type a regexp and use ‘S-TAB’ to see what it matches (and then choose one of the matching candidates to input, if you want).Does Emacs support lookahead/lookbehind?
Does Emacs support possessive quantifiers such as ?+, *+, ++ ?
The escape sequence \cC represents any character of category “C”, and according to Emacs documentation invoked by “M-x describe-categories”, \c6 ought to match any digit. Yet the ASCII digits 0-9 are not matched by \c6. Is this an error, or just something on the to-do list?