This is Info file elisp, produced by Makeinfo version 1.68 from the input file elisp.texi. INFO-DIR-SECTION Editors START-INFO-DIR-ENTRY * Elisp: (elisp). The Emacs Lisp Reference Manual. END-INFO-DIR-ENTRY This version is the edition 2.5 of the GNU Emacs Lisp Reference Manual. It corresponds to Emacs Version 20.3 Published by the Free Software Foundation 59 Temple Place, Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English.  File: elisp, Node: Lisp and Coding Systems, Next: User-Chosen Coding Systems, Prev: Encoding and I/O, Up: Coding Systems Coding Systems in Lisp ---------------------- Here are Lisp facilities for working with coding systems; - Function: coding-system-list &optional BASE-ONLY This function returns a list of all coding system names (symbols). If BASE-ONLY is non-`nil', the value includes only the base coding systems. Otherwise, it includes variant coding systems as well. - Function: coding-system-p OBJECT This function returns `t' if OBJECT is a coding system name. - Function: check-coding-system CODING-SYSTEM This function checks the validity of CODING-SYSTEM. If that is valid, it returns CODING-SYSTEM. Otherwise it signals an error with condition `coding-system-error'. - Function: coding-system-change-eol-conversion CODING-SYSTEM EOL-TYPE This function returns a coding system which is like CODING-SYSTEM except for its eol conversion, which is specified by `eol-type'. EOL-TYPE should be `unix', `dos', `mac', or `nil'. If it is `nil', the returned coding system determines the end-of-line conversion from the data. - Function: coding-system-change-text-conversion EOL-CODING TEXT-CODING This function returns a coding system which uses the end-of-line conversion of EOL-CODING, and the text conversion of TEXT-CODING. If TEXT-CODING is `nil', it returns `undecided', or one of its variants according to EOL-CODING. - Function: find-coding-systems-region FROM TO This function returns a list of coding systems that could be used to encode a text between FROM and TO. All coding systems in the list can safely encode any multibyte characters in that portion of the text. If the text contains no multibyte characters, the function returns the list `(undecided)'. - Function: find-coding-systems-string STRING This function returns a list of coding systems that could be used to encode the text of STRING. All coding systems in the list can safely encode any multibyte characters in STRING. If the text contains no multibyte characters, this returns the list `(undecided)'. - Function: find-coding-systems-for-charsets CHARSETS This function returns a list of coding systems that could be used to encode all the character sets in the list CHARSETS. - Function: detect-coding-region START END &optional HIGHEST This function chooses a plausible coding system for decoding the text from START to END. This text should be "raw bytes" (*note Explicit Encoding::.). Normally this function returns a list of coding systems that could handle decoding the text that was scanned. They are listed in order of decreasing priority. But if HIGHEST is non-`nil', then the return value is just one coding system, the one that is highest in priority. If the region contains only ASCII characters, the value is `undecided' or `(undecided)'. - Function: detect-coding-string STRING HIGHEST This function is like `detect-coding-region' except that it operates on the contents of STRING instead of bytes in the buffer. *Note Process Information::, for how to examine or set the coding systems used for I/O to a subprocess.  File: elisp, Node: User-Chosen Coding Systems, Next: Default Coding Systems, Prev: Lisp and Coding Systems, Up: Coding Systems User-Chosen Coding Systems -------------------------- - Function: select-safe-coding-system FROM TO &optional PREFERRED-CODING-SYSTEM This function selects a coding system for encoding the text between FROM and TO, asking the user to choose if necessary. The optional argument PREFERRED-CODING-SYSTEM specifies a coding system to try first. If that one can handle the text in the specified region, then it is used. If this argument is omitted, the current buffer's value of `buffer-file-coding-system' is tried first. If the region contains some multibyte characters that the preferred coding system cannot encode, this function asks the user to choose from a list of coding systems which can encode the text, and returns the user's choice. One other kludgy feature: if FROM is a string, the string is the target text, and TO is ignored. Here are two functions you can use to let the user specify a coding system, with completion. *Note Completion::. - Function: read-coding-system PROMPT &optional DEFAULT This function reads a coding system using the minibuffer, prompting with string PROMPT, and returns the coding system name as a symbol. If the user enters null input, DEFAULT specifies which coding system to return. It should be a symbol or a string. - Function: read-non-nil-coding-system PROMPT This function reads a coding system using the minibuffer, prompting with string PROMPT, and returns the coding system name as a symbol. If the user tries to enter null input, it asks the user to try again. *Note Coding Systems::.  File: elisp, Node: Default Coding Systems, Next: Specifying Coding Systems, Prev: User-Chosen Coding Systems, Up: Coding Systems Default Coding Systems ---------------------- This section describes variables that specify the default coding system for certain files or when running certain subprograms, and the function that I/O operations use to access them. The idea of these variables is that you set them once and for all to the defaults you want, and then do not change them again. To specify a particular coding system for a particular operation in a Lisp program, don't change these variables; instead, override them using `coding-system-for-read' and `coding-system-for-write' (*note Specifying Coding Systems::.). - Variable: file-coding-system-alist This variable is an alist that specifies the coding systems to use for reading and writing particular files. Each element has the form `(PATTERN . CODING)', where PATTERN is a regular expression that matches certain file names. The element applies to file names that match PATTERN. The CDR of the element, CODING, should be either a coding system, a cons cell containing two coding systems, or a function symbol. If VAL is a coding system, that coding system is used for both reading the file and writing it. If VAL is a cons cell containing two coding systems, its CAR specifies the coding system for decoding, and its CDR specifies the coding system for encoding. If VAL is a function symbol, the function must return a coding system or a cons cell containing two coding systems. This value is used as described above. - Variable: process-coding-system-alist This variable is an alist specifying which coding systems to use for a subprocess, depending on which program is running in the subprocess. It works like `file-coding-system-alist', except that PATTERN is matched against the program name used to start the subprocess. The coding system or systems specified in this alist are used to initialize the coding systems used for I/O to the subprocess, but you can specify other coding systems later using `set-process-coding-system'. *Warning:* Coding systems such as `undecided' which determine the coding system from the data do not work entirely reliably with asynchronous subprocess output. This is because Emacs handles asynchronous subprocess output in batches, as it arrives. If the coding system leaves the character code conversion unspecified, or leaves the end-of-line conversion unspecified, Emacs must try to detect the proper conversion from one batch at a time, and this does not always work. Therefore, with an asynchronous subprocess, if at all possible, use a coding system which determines both the character code conversion and the end of line conversion--that is, one like `latin-1-unix', rather than `undecided' or `latin-1'. - Variable: network-coding-system-alist This variable is an alist that specifies the coding system to use for network streams. It works much like `file-coding-system-alist', with the difference that the PATTERN in an element may be either a port number or a regular expression. If it is a regular expression, it is matched against the network service name used to open the network stream. - Variable: default-process-coding-system This variable specifies the coding systems to use for subprocess (and network stream) input and output, when nothing else specifies what to do. The value should be a cons cell of the form `(INPUT-CODING . OUTPUT-CODING)'. Here INPUT-CODING applies to input from the subprocess, and OUTPUT-CODING applies to output to it. - Function: find-operation-coding-system OPERATION &rest ARGUMENTS This function returns the coding system to use (by default) for performing OPERATION with ARGUMENTS. The value has this form: (DECODING-SYSTEM ENCODING-SYSTEM) The first element, DECODING-SYSTEM, is the coding system to use for decoding (in case OPERATION does decoding), and ENCODING-SYSTEM is the coding system for encoding (in case OPERATION does encoding). The argument OPERATION should be an Emacs I/O primitive: `insert-file-contents', `write-region', `call-process', `call-process-region', `start-process', or `open-network-stream'. The remaining arguments should be the same arguments that might be given to that I/O primitive. Depending on which primitive, one of those arguments is selected as the "target". For example, if OPERATION does file I/O, whichever argument specifies the file name is the target. For subprocess primitives, the process name is the target. For `open-network-stream', the target is the service name or port number. This function looks up the target in `file-coding-system-alist', `process-coding-system-alist', or `network-coding-system-alist', depending on OPERATION. *Note Default Coding Systems::.  File: elisp, Node: Specifying Coding Systems, Next: Explicit Encoding, Prev: Default Coding Systems, Up: Coding Systems Specifying a Coding System for One Operation -------------------------------------------- You can specify the coding system for a specific operation by binding the variables `coding-system-for-read' and/or `coding-system-for-write'. - Variable: coding-system-for-read If this variable is non-`nil', it specifies the coding system to use for reading a file, or for input from a synchronous subprocess. It also applies to any asynchronous subprocess or network stream, but in a different way: the value of `coding-system-for-read' when you start the subprocess or open the network stream specifies the input decoding method for that subprocess or network stream. It remains in use for that subprocess or network stream unless and until overridden. The right way to use this variable is to bind it with `let' for a specific I/O operation. Its global value is normally `nil', and you should not globally set it to any other value. Here is an example of the right way to use the variable: ;; Read the file with no character code conversion. ;; Assume CRLF represents end-of-line. (let ((coding-system-for-write 'emacs-mule-dos)) (insert-file-contents filename)) When its value is non-`nil', `coding-system-for-read' takes precedence over all other methods of specifying a coding system to use for input, including `file-coding-system-alist', `process-coding-system-alist' and `network-coding-system-alist'. - Variable: coding-system-for-write This works much like `coding-system-for-read', except that it applies to output rather than input. It affects writing to files, subprocesses, and net connections. When a single operation does both input and output, as do `call-process-region' and `start-process', both `coding-system-for-read' and `coding-system-for-write' affect it. - Variable: inhibit-eol-conversion When this variable is non-`nil', no end-of-line conversion is done, no matter which coding system is specified. This applies to all the Emacs I/O and subprocess primitives, and to the explicit encoding and decoding functions (*note Explicit Encoding::.).  File: elisp, Node: Explicit Encoding, Next: Terminal I/O Encoding, Prev: Specifying Coding Systems, Up: Coding Systems Explicit Encoding and Decoding ------------------------------ All the operations that transfer text in and out of Emacs have the ability to use a coding system to encode or decode the text. You can also explicitly encode and decode text using the functions in this section. The result of encoding, and the input to decoding, are not ordinary text. They are "raw bytes"--bytes that represent text in the same way that an external file would. When a buffer contains raw bytes, it is most natural to mark that buffer as using unibyte representation, using `set-buffer-multibyte' (*note Selecting a Representation::.), but this is not required. If the buffer's contents are only temporarily raw, leave the buffer multibyte, which will be correct after you decode them. The usual way to get raw bytes in a buffer, for explicit decoding, is to read them from a file with `insert-file-contents-literally' (*note Reading from Files::.) or specify a non-`nil' RAWFILE argument when visiting a file with `find-file-noselect'. The usual way to use the raw bytes that result from explicitly encoding text is to copy them to a file or process--for example, to write them with `write-region' (*note Writing to Files::.), and suppress encoding for that `write-region' call by binding `coding-system-for-write' to `no-conversion'. Raw bytes sometimes contain overlong byte-sequences that look like a proper multibyte character plus extra bytes containing trailing codes. For most purposes, Emacs treats such a sequence in a buffer or string as a single character, and if you look at its character code, you get the value that corresponds to the multibyte character sequence--the extra bytes are disregarded. This behavior is not quite clean, but raw bytes are used only in limited places in Emacs, so as a practical matter problems can be avoided. - Function: encode-coding-region START END CODING-SYSTEM This function encodes the text from START to END according to coding system CODING-SYSTEM. The encoded text replaces the original text in the buffer. The result of encoding is "raw bytes," but the buffer remains multibyte if it was multibyte before. - Function: encode-coding-string STRING CODING-SYSTEM This function encodes the text in STRING according to coding system CODING-SYSTEM. It returns a new string containing the encoded text. The result of encoding is a unibyte string of "raw bytes." - Function: decode-coding-region START END CODING-SYSTEM This function decodes the text from START to END according to coding system CODING-SYSTEM. The decoded text replaces the original text in the buffer. To make explicit decoding useful, the text before decoding ought to be "raw bytes." - Function: decode-coding-string STRING CODING-SYSTEM This function decodes the text in STRING according to coding system CODING-SYSTEM. It returns a new string containing the decoded text. To make explicit decoding useful, the contents of STRING ought to be "raw bytes."  File: elisp, Node: Terminal I/O Encoding, Next: MS-DOS File Types, Prev: Explicit Encoding, Up: Coding Systems Terminal I/O Encoding --------------------- Emacs can decode keyboard input using a coding system, and encode terminal output. This is useful for terminals that transmit or display text using a particular encoding such as Latin-1. Emacs does not set `last-coding-system-used' for encoding or decoding for the terminal. - Function: keyboard-coding-system This function returns the coding system that is in use for decoding keyboard input--or `nil' if no coding system is to be used. - Function: set-keyboard-coding-system CODING-SYSTEM This function specifies CODING-SYSTEM as the coding system to use for decoding keyboard input. If CODING-SYSTEM is `nil', that means do not decode keyboard input. - Function: terminal-coding-system This function returns the coding system that is in use for encoding terminal output--or `nil' for no encoding. - Function: set-terminal-coding-system CODING-SYSTEM This function specifies CODING-SYSTEM as the coding system to use for encoding terminal output. If CODING-SYSTEM is `nil', that means do not encode terminal output.  File: elisp, Node: MS-DOS File Types, Prev: Terminal I/O Encoding, Up: Coding Systems MS-DOS File Types ----------------- Emacs on MS-DOS and on MS-Windows recognizes certain file names as text files or binary files. By "binary file" we mean a file of literal byte values that are not necessary meant to be characters. Emacs does no end-of-line conversion and no character code conversion for a binary file. Meanwhile, when you create a new file which is marked by its name as a "text file", Emacs uses DOS end-of-line conversion. - Variable: buffer-file-type This variable, automatically buffer-local in each buffer, records the file type of the buffer's visited file. When a buffer does not specify a coding system with `buffer-file-coding-system', this variable is used to determine which coding system to use when writing the contents of the buffer. It should be `nil' for text, `t' for binary. If it is `t', the coding system is `no-conversion'. Otherwise, `undecided-dos' is used. Normally this variable is set by visiting a file; it is set to `nil' if the file was visited without any actual conversion. - User Option: file-name-buffer-file-type-alist This variable holds an alist for recognizing text and binary files. Each element has the form (REGEXP . TYPE), where REGEXP is matched against the file name, and TYPE may be `nil' for text, `t' for binary, or a function to call to compute which. If it is a function, then it is called with a single argument (the file name) and should return `t' or `nil'. Emacs when running on MS-DOS or MS-Windows checks this alist to decide which coding system to use when reading a file. For a text file, `undecided-dos' is used. For a binary file, `no-conversion' is used. If no element in this alist matches a given file name, then `default-buffer-file-type' says how to treat the file. - User Option: default-buffer-file-type This variable says how to handle files for which `file-name-buffer-file-type-alist' says nothing about the type. If this variable is non-`nil', then these files are treated as binary: the coding system `no-conversion' is used. Otherwise, nothing special is done for them--the coding system is deduced solely from the file contents, in the usual Emacs fashion.  File: elisp, Node: Input Methods, Prev: Coding Systems, Up: Non-ASCII Characters Input Methods ============= "Input methods" provide convenient ways of entering non-ASCII characters from the keyboard. Unlike coding systems, which translate non-ASCII characters to and from encodings meant to be read by programs, input methods provide human-friendly commands. (*Note Input Methods: (emacs)Input Methods, for information on how users use input methods to enter text.) How to define input methods is not yet documented in this manual, but here we describe how to use them. Each input method has a name, which is currently a string; in the future, symbols may also be usable as input method names. - Variable: current-input-method This variable holds the name of the input method now active in the current buffer. (It automatically becomes local in each buffer when set in any fashion.) It is `nil' if no input method is active in the buffer now. - Variable: default-input-method This variable holds the default input method for commands that choose an input method. Unlike `current-input-method', this variable is normally global. - Function: set-input-method INPUT-METHOD This function activates input method INPUT-METHOD for the current buffer. It also sets `default-input-method' to INPUT-METHOD. If INPUT-METHOD is `nil', this function deactivates any input method for the current buffer. - Function: read-input-method-name PROMPT &optional DEFAULT INHIBIT-NULL This function reads an input method name with the minibuffer, prompting with PROMPT. If DEFAULT is non-`nil', that is returned by default, if the user enters empty input. However, if INHIBIT-NULL is non-`nil', empty input signals an error. The returned value is a string. - Variable: input-method-alist This variable defines all the supported input methods. Each element defines one input method, and should have the form: (INPUT-METHOD LANGUAGE-ENV ACTIVATE-FUNC TITLE DESCRIPTION ARGS...) Here INPUT-METHOD is the input method name, a string; LANGUAGE-ENV is another string, the name of the language environment this input method is recommended for. (That serves only for documentation purposes.) TITLE is a string to display in the mode line while this method is active. DESCRIPTION is a string describing this method and what it is good for. ACTIVATE-FUNC is a function to call to activate this method. The ARGS, if any, are passed as arguments to ACTIVATE-FUNC. All told, the arguments to ACTIVATE-FUNC are INPUT-METHOD and the ARGS. The fundamental interface to input methods is through the variable `input-method-function'. *Note Reading One Event::.  File: elisp, Node: Searching and Matching, Next: Syntax Tables, Prev: Non-ASCII Characters, Up: Top Searching and Matching ********************** GNU Emacs provides two ways to search through a buffer for specified text: exact string searches and regular expression searches. After a regular expression search, you can examine the "match data" to determine which text matched the whole regular expression or various portions of it. * Menu: * String Search:: Search for an exact match. * Regular Expressions:: Describing classes of strings. * Regexp Search:: Searching for a match for a regexp. * POSIX Regexps:: Searching POSIX-style for the longest match. * Search and Replace:: Internals of `query-replace'. * Match Data:: Finding out which part of the text matched various parts of a regexp, after regexp search. * Searching and Case:: Case-independent or case-significant searching. * Standard Regexps:: Useful regexps for finding sentences, pages,... The `skip-chars...' functions also perform a kind of searching. *Note Skipping Characters::.  File: elisp, Node: String Search, Next: Regular Expressions, Up: Searching and Matching Searching for Strings ===================== These are the primitive functions for searching through the text in a buffer. They are meant for use in programs, but you may call them interactively. If you do so, they prompt for the search string; LIMIT and NOERROR are set to `nil', and REPEAT is set to 1. These search functions convert the search string to multibyte if the buffer is multibyte; they convert the search string to unibyte if the buffer is unibyte. *Note Text Representations::. - Command: search-forward STRING &optional LIMIT NOERROR REPEAT This function searches forward from point for an exact match for STRING. If successful, it sets point to the end of the occurrence found, and returns the new value of point. If no match is found, the value and side effects depend on NOERROR (see below). In the following example, point is initially at the beginning of the line. Then `(search-forward "fox")' moves point after the last letter of `fox': ---------- Buffer: foo ---------- -!-The quick brown fox jumped over the lazy dog. ---------- Buffer: foo ---------- (search-forward "fox") => 20 ---------- Buffer: foo ---------- The quick brown fox-!- jumped over the lazy dog. ---------- Buffer: foo ---------- The argument LIMIT specifies the upper bound to the search. (It must be a position in the current buffer.) No match extending after that position is accepted. If LIMIT is omitted or `nil', it defaults to the end of the accessible portion of the buffer. What happens when the search fails depends on the value of NOERROR. If NOERROR is `nil', a `search-failed' error is signaled. If NOERROR is `t', `search-forward' returns `nil' and does nothing. If NOERROR is neither `nil' nor `t', then `search-forward' moves point to the upper bound and returns `nil'. (It would be more consistent now to return the new position of point in that case, but some existing programs may depend on a value of `nil'.) If REPEAT is supplied (it must be a positive number), then the search is repeated that many times (each time starting at the end of the previous time's match). If these successive searches succeed, the function succeeds, moving point and returning its new value. Otherwise the search fails. - Command: search-backward STRING &optional LIMIT NOERROR REPEAT This function searches backward from point for STRING. It is just like `search-forward' except that it searches backwards and leaves point at the beginning of the match. - Command: word-search-forward STRING &optional LIMIT NOERROR REPEAT This function searches forward from point for a "word" match for STRING. If it finds a match, it sets point to the end of the match found, and returns the new value of point. Word matching regards STRING as a sequence of words, disregarding punctuation that separates them. It searches the buffer for the same sequence of words. Each word must be distinct in the buffer (searching for the word `ball' does not match the word `balls'), but the details of punctuation and spacing are ignored (searching for `ball boy' does match `ball. Boy!'). In this example, point is initially at the beginning of the buffer; the search leaves it between the `y' and the `!'. ---------- Buffer: foo ---------- -!-He said "Please! Find the ball boy!" ---------- Buffer: foo ---------- (word-search-forward "Please find the ball, boy.") => 35 ---------- Buffer: foo ---------- He said "Please! Find the ball boy-!-!" ---------- Buffer: foo ---------- If LIMIT is non-`nil' (it must be a position in the current buffer), then it is the upper bound to the search. The match found must not extend after that position. If NOERROR is `nil', then `word-search-forward' signals an error if the search fails. If NOERROR is `t', then it returns `nil' instead of signaling an error. If NOERROR is neither `nil' nor `t', it moves point to LIMIT (or the end of the buffer) and returns `nil'. If REPEAT is non-`nil', then the search is repeated that many times. Point is positioned at the end of the last match. - Command: word-search-backward STRING &optional LIMIT NOERROR REPEAT This function searches backward from point for a word match to STRING. This function is just like `word-search-forward' except that it searches backward and normally leaves point at the beginning of the match.  File: elisp, Node: Regular Expressions, Next: Regexp Search, Prev: String Search, Up: Searching and Matching Regular Expressions =================== A "regular expression" ("regexp", for short) is a pattern that denotes a (possibly infinite) set of strings. Searching for matches for a regexp is a very powerful operation. This section explains how to write regexps; the following section says how to search for them. * Menu: * Syntax of Regexps:: Rules for writing regular expressions. * Regexp Example:: Illustrates regular expression syntax.  File: elisp, Node: Syntax of Regexps, Next: Regexp Example, Up: Regular Expressions Syntax of Regular Expressions ----------------------------- Regular expressions have a syntax in which a few characters are special constructs and the rest are "ordinary". An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are `.', `*', `+', `?', `[', `]', `^', `$', and `\'; no new special characters will be defined in the future. Any other character appearing in a regular expression is ordinary, unless a `\' precedes it. For example, `f' is not a special character, so it is ordinary, and therefore `f' is a regular expression that matches the string `f' and no other string. (It does *not* match the string `ff'.) Likewise, `o' is a regular expression that matches only `o'. Any two regular expressions A and B can be concatenated. The result is a regular expression that matches a string if A matches some amount of the beginning of that string and B matches the rest of the string. As a simple example, we can concatenate the regular expressions `f' and `o' to get the regular expression `fo', which matches only the string `fo'. Still trivial. To do something more powerful, you need to use one of the special characters. Here is a list of them: `.' (Period) is a special character that matches any single character except a newline. Using concatenation, we can make regular expressions like `a.b', which matches any three-character string that begins with `a' and ends with `b'. `*' is not a construct by itself; it is a postfix operator that means to match the preceding regular expression repetitively as many times as possible. Thus, `o*' matches any number of `o's (including no `o's). `*' always applies to the *smallest* possible preceding expression. Thus, `fo*' has a repeating `o', not a repeating `fo'. It matches `f', `fo', `foo', and so on. The matcher processes a `*' construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the `*'-modified construct in the hope that that will make it possible to match the rest of the pattern. For example, in matching `ca*ar' against the string `caaar', the `a*' first tries to match all three `a's; but the rest of the pattern is `ar' and there is only `r' left to match, so this try fails. The next alternative is for `a*' to match only two `a's. With this choice, the rest of the regexp matches successfully. Nested repetition operators can be extremely slow if they specify backtracking loops. For example, it could take hours for the regular expression `\(x+y*\)*a' to try to match the sequence `xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz', before it ultimately fails. The slowness is because Emacs must try each imaginable way of grouping the 35 `x's before concluding that none of them can work. To make sure your regular expressions run fast, check nested repetitions carefully. `+' is a postfix operator, similar to `*' except that it must match the preceding expression at least once. So, for example, `ca+r' matches the strings `car' and `caaaar' but not the string `cr', whereas `ca*r' matches all three strings. `?' is a postfix operator, similar to `*' except that it must match the preceding expression either once or not at all. For example, `ca?r' matches `car' or `cr'; nothing else. `[ ... ]' is a "character alternative", which begins with `[' and is terminated by `]'. In the simplest case, the characters between the two brackets are what this character alternative can match. Thus, `[ad]' matches either one `a' or one `d', and `[ad]*' matches any string composed of just `a's and `d's (including the empty string), from which it follows that `c[ad]*r' matches `cr', `car', `cdr', `caddaar', etc. You can also include character ranges in a character alternative, by writing the starting and ending characters with a `-' between them. Thus, `[a-z]' matches any lower-case ASCII letter. Ranges may be intermixed freely with individual characters, as in `[a-z$%.]', which matches any lower case ASCII letter or `$', `%' or period. You cannot always match all non-ASCII characters with the regular expression `[\200-\377]'. This works when searching a unibyte buffer or string (*note Text Representations::.), but not in a multibyte buffer or string, because many non-ASCII characters have codes above octal 0377. However, the regular expression `[^\000-\177]' does match all non-ASCII characters, in both multibyte and unibyte representations, because only the ASCII characters are excluded. The beginning and end of a range must be in the same character set (*note Character Sets::.). Thus, `[a-\x8e0]' is invalid because `a' is in the ASCII character set but the character 0x8e0 (`a' with grave accent) is in the Emacs character set for Latin-1. Note that the usual regexp special characters are not special inside a character alternative. A completely different set of characters are special inside character alternatives: `]', `-' and `^'. To include a `]' in a character alternative, you must make it the first character. For example, `[]a]' matches `]' or `a'. To include a `-', write `-' as the first or last character of the character alternative, or put it after a range. Thus, `[]-]' matches both `]' and `-'. To include `^' in a character alternative, put it anywhere but at the beginning. `[^ ... ]' `[^' begins a "complemented character alternative", which matches any character except the ones specified. Thus, `[^a-z0-9A-Z]' matches all characters *except* letters and digits. `^' is not special in a character alternative unless it is the first character. The character following the `^' is treated as if it were first (in other words, `-' and `]' are not special there). A complemented character alternative can match a newline, unless newline is mentioned as one of the characters not to match. This is in contrast to the handling of regexps in programs such as `grep'. `^' is a special character that matches the empty string, but only at the beginning of a line in the text being matched. Otherwise it fails to match anything. Thus, `^foo' matches a `foo' that occurs at the beginning of a line. When matching a string instead of a buffer, `^' matches at the beginning of the string or after a newline character `\n'. `$' is similar to `^' but matches only at the end of a line. Thus, `x+$' matches a string of one `x' or more at the end of a line. When matching a string instead of a buffer, `$' matches at the end of the string or before a newline character `\n'. `\' has two functions: it quotes the special characters (including `\'), and it introduces additional special constructs. Because `\' quotes special characters, `\$' is a regular expression that matches only `$', and `\[' is a regular expression that matches only `[', and so on. Note that `\' also has special meaning in the read syntax of Lisp strings (*note String Type::.), and must be quoted with `\'. For example, the regular expression that matches the `\' character is `\\'. To write a Lisp string that contains the characters `\\', Lisp syntax requires you to quote each `\' with another `\'. Therefore, the read syntax for a regular expression matching `\' is `"\\\\"'. *Please note:* For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, `*foo' treats `*' as ordinary since there is no preceding expression on which the `*' can act. It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears. For the most part, `\' followed by any character matches only that character. However, there are several exceptions: two-character sequences starting with `\' which have special meanings. (The second character in such a sequence is always ordinary when used on its own.) Here is a table of `\' constructs. `\|' specifies an alternative. Two regular expressions A and B with `\|' in between form an expression that matches anything that either A or B matches. Thus, `foo\|bar' matches either `foo' or `bar' but no other string. `\|' applies to the largest possible surrounding expressions. Only a surrounding `\( ... \)' grouping can limit the grouping power of `\|'. Full backtracking capability exists to handle multiple uses of `\|'. `\( ... \)' is a grouping construct that serves three purposes: 1. To enclose a set of `\|' alternatives for other operations. Thus, the regular expression `\(foo\|bar\)x' matches either `foox' or `barx'. 2. To enclose a complicated expression for the postfix operators `*', `+' and `?' to operate on. Thus, `ba\(na\)*' matches `ba', `bana', `banana', `bananana', etc., with any number (zero or more) of `na' strings. 3. To record a matched substring for future reference. This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that happens to be assigned as a second meaning to the same `\( ... \)' construct because there is no conflict in practice between the two meanings. Here is an explanation of this feature: `\DIGIT' matches the same text that matched the DIGITth occurrence of a `\( ... \)' construct. In other words, after the end of a `\( ... \)' construct, the matcher remembers the beginning and end of the text matched by that construct. Then, later on in the regular expression, you can use `\' followed by DIGIT to match that same text, whatever it may have been. The strings matching the first nine `\( ... \)' constructs appearing in a regular expression are assigned numbers 1 through 9 in the order that the open parentheses appear in the regular expression. So you can use `\1' through `\9' to refer to the text matched by the corresponding `\( ... \)' constructs. For example, `\(.*\)\1' matches any newline-free string that is composed of two identical halves. The `\(.*\)' matches the first half, which may be anything, but the `\1' that follows must match the same exact text. `\w' matches any word-constituent character. The editor syntax table determines which characters these are. *Note Syntax Tables::. `\W' matches any character that is not a word constituent. `\sCODE' matches any character whose syntax is CODE. Here CODE is a character that represents a syntax code: thus, `w' for word constituent, `-' for whitespace, `(' for open parenthesis, etc. To represent whitespace syntax, use either `-' or a space character. *Note Syntax Class Table::, for a list of syntax codes and the characters that stand for them. `\SCODE' matches any character whose syntax is not CODE. The following regular expression constructs match the empty string--that is, they don't use up any characters--but whether they match depends on the context. `\`' matches the empty string, but only at the beginning of the buffer or string being matched against. `\'' matches the empty string, but only at the end of the buffer or string being matched against. `\=' matches the empty string, but only at point. (This construct is not defined when matching against a string.) `\b' matches the empty string, but only at the beginning or end of a word. Thus, `\bfoo\b' matches any occurrence of `foo' as a separate word. `\bballs?\b' matches `ball' or `balls' as a separate word. `\b' matches at the beginning or end of the buffer regardless of what text appears next to it. `\B' matches the empty string, but *not* at the beginning or end of a word. `\<' matches the empty string, but only at the beginning of a word. `\<' matches at the beginning of the buffer only if a word-constituent character follows. `\>' matches the empty string, but only at the end of a word. `\>' matches at the end of the buffer only if the contents end with a word-constituent character. Not every string is a valid regular expression. For example, a string with unbalanced square brackets is invalid (with a few exceptions, such as `[]]'), and so is a string that ends with a single `\'. If an invalid regular expression is passed to any of the search functions, an `invalid-regexp' error is signaled. - Function: regexp-quote STRING This function returns a regular expression string that matches exactly STRING and nothing else. This allows you to request an exact string match when calling a function that wants a regular expression. (regexp-quote "^The cat$") => "\\^The cat\\$" One use of `regexp-quote' is to combine an exact string match with context described as a regular expression. For example, this searches for the string that is the value of STRING, surrounded by whitespace: (re-search-forward (concat "\\s-" (regexp-quote string) "\\s-")) - Function: regexp-opt STRINGS &optional PAREN This function returns an efficient regular expression that will match any of the strings STRINGS. This is useful when you need to make matching or searching as fast as possible--for example, for Font Lock mode. If the optional argument PAREN is non-`nil', then the returned regular expression is always enclosed by at least one parentheses-grouping construct. This simplified definition of `regexp-opt' produces a regular expression which is equivalent to the actual value (but not as efficient): (defun regexp-opt (strings paren) (let ((open-paren (if paren "\\(" "")) (close-paren (if paren "\\)" ""))) (concat open-paren (mapconcat 'regexp-quote strings "\\|") close-paren))) - Function: regexp-opt-depth REGEXP This function returns the total number of grouping constructs (parenthesized expressions) in REGEXP.  File: elisp, Node: Regexp Example, Prev: Syntax of Regexps, Up: Regular Expressions Complex Regexp Example ---------------------- Here is a complicated regexp, used by Emacs to recognize the end of a sentence together with any whitespace that follows. It is the value of the variable `sentence-end'. First, we show the regexp as a string in Lisp syntax to distinguish spaces from tab characters. The string constant begins and ends with a double-quote. `\"' stands for a double-quote as part of the string, `\\' for a backslash as part of the string, `\t' for a tab and `\n' for a newline. "[.?!][]\"')}]*\\($\\| $\\|\t\\| \\)[ \t\n]*" In contrast, if you evaluate the variable `sentence-end', you will see the following: sentence-end => "[.?!][]\"')}]*\\($\\| $\\| \\| \\)[ ]*" In this output, tab and newline appear as themselves. This regular expression contains four parts in succession and can be deciphered as follows: `[.?!]' The first part of the pattern is a character alternative that matches any one of three characters: period, question mark, and exclamation mark. The match must begin with one of these three characters. `[]\"')}]*' The second part of the pattern matches any closing braces and quotation marks, zero or more of them, that may follow the period, question mark or exclamation mark. The `\"' is Lisp syntax for a double-quote in a string. The `*' at the end indicates that the immediately preceding regular expression (a character alternative, in this case) may be repeated zero or more times. `\\($\\| $\\|\t\\| \\)' The third part of the pattern matches the whitespace that follows the end of a sentence: the end of a line (optionally with a space), or a tab, or two spaces. The double backslashes mark the parentheses and vertical bars as regular expression syntax; the parentheses delimit a group and the vertical bars separate alternatives. The dollar sign is used to match the end of a line. `[ \t\n]*' Finally, the last part of the pattern matches any additional whitespace beyond the minimum needed to end a sentence.