Info Catalog emacs: Symbol Search emacs: Search emacs: Regexps

emacs: Regexp Search

 
 15.5 Regular Expression Search
 ==============================
 
 A “regular expression” (or “regexp” for short) is a pattern that denotes
 a class of alternative strings to match.  Emacs provides both
 incremental and nonincremental ways to search for a match for a regexp.
 The syntax of regular expressions is explained in the next section.
 
 ‘C-M-s’
      Begin incremental regexp search (‘isearch-forward-regexp’).
 ‘C-M-r’
      Begin reverse incremental regexp search
      (‘isearch-backward-regexp’).
 
    Incremental search for a regexp is done by typing ‘C-M-s’
 (‘isearch-forward-regexp’), by invoking ‘C-s’ with a prefix argument
 (whose value does not matter), or by typing ‘M-r’ within a forward
 incremental search.  This command reads a search string incrementally
 just like ‘C-s’, but it treats the search string as a regexp rather than
 looking for an exact match against the text in the buffer.  Each time
 you add text to the search string, you make the regexp longer, and the
 new regexp is searched for.  To search backward for a regexp, use
 ‘C-M-r’ (‘isearch-backward-regexp’), ‘C-r’ with a prefix argument, or
 ‘M-r’ within a backward incremental search.
 
    All of the special key sequences in an ordinary incremental search
 (Special Isearch) do similar things in an incremental regexp
 search.  For instance, typing ‘C-s’ immediately after starting the
 search retrieves the last incremental search regexp used and searches
 forward for it.  Incremental regexp and non-regexp searches have
 independent defaults.  They also have separate search rings, which you
 can access with ‘M-p’ and ‘M-n’.  The maximum number of search regexps
 saved in the search ring is determined by the value of
 ‘regexp-search-ring-max’, 16 by default.
 
    Unlike ordinary incremental search, incremental regexp search does
 not use lax space matching by default.  To toggle this feature use ‘M-s
 <SPC>’ (‘isearch-toggle-lax-whitespace’).  Then any <SPC> typed in
 incremental regexp search will match any sequence of one or more
 whitespace characters.  The variable ‘search-whitespace-regexp’
 specifies the regexp for the lax space matching.  Special
 Isearch.
 
    Also unlike ordinary incremental search, incremental regexp search
 cannot use character folding (Lax Search).  (If you toggle
 character folding during incremental regexp search with ‘M-s '’, the
 search becomes a non-regexp search and the search pattern you typed is
 interpreted as a literal string.)
 
    In some cases, adding characters to the regexp in an incremental
 regexp search can make the cursor move back and start again.  For
 example, if you have searched for ‘foo’ and you add ‘\|bar’, the cursor
 backs up in case the first ‘bar’ precedes the first ‘foo’.  
 Regexps.
 
    Forward and backward regexp search are not symmetrical, because
 regexp matching in Emacs always operates forward, starting with the
 beginning of the regexp.  Thus, forward regexp search scans forward,
 trying a forward match at each possible starting position.  Backward
 regexp search scans backward, trying a forward match at each possible
 starting position.  These search methods are not mirror images.
 
    Nonincremental search for a regexp is done with the commands
 ‘re-search-forward’ and ‘re-search-backward’.  You can invoke these with
 ‘M-x’, or by way of incremental regexp search with ‘C-M-s <RET>’ and
 ‘C-M-r <RET>’.  When you invoke these commands with ‘M-x’, they search
 for the exact regexp you specify, and thus don’t support any lax-search
 features (Lax Search) except case folding.
 
    If you use the incremental regexp search commands with a prefix
 argument, they perform ordinary string search, like ‘isearch-forward’
 and ‘isearch-backward’.  Incremental Search.
 

Info Catalog emacs: Symbol Search emacs: Search emacs: Regexps