1 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 identify the text matched by parts of the regular expression by examining the match data.

1.1 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.

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 positioned at the beginning of the line. Then (search-forward "fox") is evaluated in the minibuffer and point is left after the last letter of ‘fox’:

---------- Buffer: foo ----------
∗The quick brown fox jumped over the lazy dog.
---------- Buffer: foo ----------

(search-forward "fox")
     ⇒ t

---------- 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 programs may depend on a value of 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: 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.

A word search differs from a simple string search in that a word search requires that the words it searches for are present as entire words (searching for the word ‘ball’ does not match the word ‘balls’), and punctuation and spacing are ignored (searching for ‘ball boy’ does match ‘ball. Boy!’).

In this example, point is first placed 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.")
     ⇒ t

---------- 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 t, then word-search-forward returns nil when a search fails, instead of signaling an error. If noerror is neither nil nor t, then word-search-forward 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.

1.2 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.

1.2.1 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 which 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 which 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 which begins with ‘a’ and ends with ‘b’.

*

is not a construct by itself; it is a suffix that means the preceding regular expression is to be repeated as many times as possible. In ‘fo*’, the ‘*’ applies to the ‘o’, so ‘fo*’ matches one ‘f’ followed by any number of ‘o’s. The case of zero ‘o’s is allowed: ‘fo*’ does match ‘f’.

‘*’ always applies to the smallest possible preceding expression. Thus, ‘fo*’ has a repeating ‘o’, not a repeating ‘fo’.

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 case that makes it possible to match the rest of the pattern. For example, 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.

+

is a suffix character similar to ‘*’ except that it must match the preceding expression at least once. So, for example, ‘ca+r’ will match the strings ‘car’ and ‘caaaar’ but not the string ‘cr’, whereas ‘ca*r’ would match all three strings.

?

is a suffix character similar to ‘*’ except that it can match the preceding expression either once or not at all. For example, ‘ca?r’ will match ‘car’ or ‘cr’; nothing else.

[ … ]

‘[’ begins a character set, which is terminated by a ‘]’. In the simplest case, the characters between the two form the set. 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.

Character ranges can also be included in a character set, by writing two characters with a ‘-’ between them. Thus, ‘[a-z]’ matches any lower case letter. Ranges may be intermixed freely with individual characters, as in ‘[a-z$%.]’, which matches any lower case letter or ‘$’, ‘%’ or a period.

Note that the usual special characters are not special any more inside a character set. A completely different set of special characters exists inside character sets: ‘]’, ‘-’ and ‘^’.

To include a ‘]’ in a character set, make it the first character. For example, ‘[]a]’ matches ‘]’ or ‘a’. To include a ‘-’, write ‘-’ as the first or last character in the range.

To include ‘^’, make it other than the first character in the set.

[^ … ]

‘[^’ begins a complement character set, 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 set unless it is the first character. The character following the ‘^’ is treated as if it were first (thus, ‘-’ and ‘]’ are not special there).

Note that a complement character set can match a newline, unless newline is mentioned as one of the characters not to match.

^

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’ which occurs at the beginning of a line.

When matching a string, ‘^’ 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, ‘$’ 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 which matches only ‘$’, and ‘\[’ is a regular expression which matches only ‘[’, and so on.

Note that ‘\’ also has special meaning in the read syntax of Lisp strings (@pxref{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; better to 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: characters which, when preceded by ‘\’, are special constructs. Such characters are always ordinary when encountered on their 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, ‘\(foo\|bar\)x’ matches either ‘foox’ or ‘barx’.
2. To enclose a complicated expression for a suffix character such as ‘*’ to operate on. Thus, ‘ba\(na\)*’ matches ‘bananana’, etc., with any (zero or more) number 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 which 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 which is matched the digitth time by a previous ‘\( … \)’ 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 mean “match the same text matched the digitth time by the ‘\( … \)’ construct.”

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.

\`

matches the empty string, provided it is at the beginning of the buffer.

\'

matches the empty string, provided it is at the end of the buffer.

\=

matches the empty string, provided it is at point.

\b

matches the empty string, provided it is 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 the empty string, provided it is not at the beginning or end of a word.

\<

matches the empty string, provided it is at the beginning of a word.

\>

matches the empty string, provided it is at the end of a word.

\w

matches any word-constituent character. The editor syntax table determines which characters these are. @xref{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 which represents a syntax code: thus, ‘w’ for word constituent, ‘-’ for whitespace, ‘(’ for open parenthesis, etc. @xref{Syntax Tables}, for a list of the codes.

\Scode

matches any character whose syntax is not code.

Not every string is a valid regular expression. For example, any string with unbalanced square brackets is invalid, 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 which 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 which is the value of string, surrounded by whitespace:

(re-search-forward
 (concat "\\s " (regexp-quote string) "\\s "))

1.2.2 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 enable you to distinguish the spaces from the 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 case, the tab and carriage return are the actual characters.

This regular expression contains four parts in succession and can be deciphered as follows:

[.?!]

The first part of the pattern consists of three characters, a period, a question mark and an exclamation mark, within square brackets. 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 set, 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, or a tab, or two spaces. The double backslashes are needed to prevent Emacs from reading the parentheses and vertical bars as part of the search pattern; the parentheses are used to mark the group and the vertical bars are used to indicated that the patterns to either side of them are alternatives. The dollar sign is used to match the end of a line. The tab character is written using ‘\t’ and the two spaces are written as themselves.

[ \t\n]*

Finally, the last part of the pattern indicates that the end of the line or the whitespace following the period, question mark or exclamation mark may, but need not, be followed by additional whitespace.

1.3 Regular Expression Searching

In GNU Emacs, you can search for the next match for a regexp either incrementally or not. Incremental search commands are described in the The GNU Emacs Manual. See Regular Expression Search in The GNU Emacs Manual. Here we describe only the search functions useful in programs. The principal one is re-search-forward.

Command: re-search-forward regexp &optional limit noerror repeat

This function searches forward in the current buffer for a string of text that is matched by the regular expression regexp. The function skips over any amount of text that is not matched by regexp, and leaves point at the end of the first string found that does match.

If the search is successful (i.e., if text matching regexp is found), then point moves to the end of that text, and the function returns the new value of point.

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, re-search-forward does nothing and returns nil. If noerror is neither nil nor t, then re-search-forward moves point to limit (or the end of the buffer) and returns nil.

If limit is non-nil (it must be a position in the current buffer), then it is the upper bound to the search. No match extending after that position is accepted.

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). The call succeeds if all these searches succeeded, and point is left at the end of the match found by the last search. Otherwise the search fails.

In the following example, point is initially located directly before the ‘T’. After evaluating the form, point is located at the end of that line (between the ‘t’ of ‘hat’ and before the newline).

---------- Buffer: foo ----------
I read "∗The cat in the hat
comes back" twice.
---------- Buffer: foo ----------

(re-search-forward "[a-z]+" nil t 5)
     ⇒ t

---------- Buffer: foo ----------
I read "The cat in the hat∗
comes back" twice.
---------- Buffer: foo ----------

Command: re-search-backward regexp &optional limit noerror repeat

This function searches backward in the current buffer for a string of text that is matched by the regular expression regexp, leaving point at the beginning of the first text found.

This function is analogous to re-search-forward, but they are not simple mirror images. re-search-forward finds the match whose beginning is as close as possible. If re-search-backward were a perfect mirror image, it would find the match whose end is as close as possible. However, in fact it finds the match whose beginning is as close as possible. The reason is that matching a regular expression at a given spot always works from beginning to end, and is done at a specified beginning position. Thus, true mirror-image behavior would require a special feature for matching regexps from end to beginning.

Function: string-match regexp string &optional start

This function returns the index of the start of the first match for the regular expression regexp in string, or nil if there is no match. If start is non-nil, the search starts at that index in string.

For example,

(string-match
 "quick" "The quick brown fox jumped quickly.")
     ⇒ 4

(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     ⇒ 27

The index of the first character of the string is 0, the index of the second character is 1, and so on.

After this function returns, the index of the first character beyond the match is available as (match-end 0). See section The Match Data.

(string-match
 "quick" "The quick brown fox jumped quickly." 8)
     ⇒ 27

(match-end 0)
     ⇒ 32

The match-beginning and match-end functions are described together; see The Match Data.

Function: looking-at regexp

This function determines whether the text in the current buffer directly following point matches the regular expression regexp. “Directly following” means precisely that: the search is “anchored” and it must succeed starting with the first character following point. The result is t if so, nil otherwise.

This function does not move point, but it updates the match data, which you can access using match-beginning or match-end. See section The Match Data.

In this example, point is located directly before the ‘T’. If it were anywhere else, the result would be nil.

---------- Buffer: foo ----------
I read "∗The cat in the hat
comes back" twice.
---------- Buffer: foo ----------

(looking-at "The cat in the hat$")
     ⇒ t

1.4 Replacement

Function: perform-replace from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map

This function is the guts of query-replace and related commands. It searches for occurrences of from-string and replaces some or all of them. If query-flag is nil, it replaces all occurrences; otherwise, it asks the user what to do about each one.

If regexp-flag is non-nil, then from-string is considered a regular expression; otherwise, it must match literally. If delimited-flag is non-nil, then only replacements surrounded by word boundaries are considered.

The argument replacements specifies what to replace occurrences with. If it is a string, that string is used. It can also be a list of strings, to be used in cyclic order.

If repeat-count is non-nil, it should be an integer, the number of occurrences to consider. In this case, perform-replace returns after considering that many occurrences.

Normally, the keymap query-replace-map defines the possible user responses. The argument map, if non-nil, is a keymap to use instead of query-replace-map.

Variable: query-replace-map

This variable holds a special keymap that defines the valid user responses for query-replace and related functions, as well as y-or-n-p and map-y-or-n-p. It is special in two ways:

• The “key bindings” are not commands, just symbols that are meaningful to the functions that use this map.
• Prefix keys are not supported; each key binding must be for a single event key sequence. This is because the functions don’t use read key sequence to get the input; instead, they read a single event and look it up “by hand.”

Here are the meaningful “bindings” for query-replace-map. Several of them are meaningful only for query-replace and friends.

act

Do take the action. The action being considered—in other words, “yes.”

skip

Do not take action for this question—in other words, “no.”

exit

Answer this question “no,” and don’t ask any more.

act-and-exit

Answer this question “yes,” and don’t ask any more.

act-and-show

Answer this question “yes,” but show the results—don’t advance yet.

automatic

Answer this question and all subsequent questions in the series with “yes,” without further user interaction.

backup

Move back to the previous place that a question was asked about.

edit

Enter a recursive edit to deal with this item—instead of any other answer.

delete-and-edit

Delete the text being considered, then enter a recursive edit to replace it.

recenter

Redisplay and center the window, then ask the same question again.

quit

Perform a quit right away. Only the y-or-n-p functions use this answer.

help

Display some help, then ask again.

1.5 The Match Data

Emacs keeps track of the positions of the start and end of segments of text found during a regular expression search. This means, for example, that you can search for a complex pattern, such as a date in an Rmail message, and extract parts of it.

Because the match data normally describe the most recent search only, you must be careful not to do another search inadvertently between the search you wish to refer back to and the use of the match data. If you can’t avoid another intervening search, you must save and restore the match data around it, to prevent it from being overwritten.

1.5.1 Simple Match Data Access

This section explains how to use the match data to find the starting point or ending point of the text that was matched by a particular search, or by a particular parenthetical subexpression of a regular expression.

Function: match-beginning count

This function returns the position of the start of text matched by the last regular expression searched for. count, a number, specifies a subexpression whose start position is the value. If count is zero, then the value is the position of the text matched by the whole regexp. If count is greater than zero, then the value is the position of the beginning of the text matched by the countth subexpression, regardless of whether it was used in the final match.

Subexpressions of a regular expression are those expressions grouped inside of parentheses, ‘\(…\)’. The countth subexpression is found by counting occurrences of ‘\(’ from the beginning of the whole regular expression. The first subexpression is numbered 1, the second 2, and so on.

The value is nil for a parenthetical grouping inside of a ‘\|’ alternative that wasn’t used in the match.

The match-end function is similar to the match-beginning function except that it returns the position of the end of the matched text.

Here is an example, with a comment showing the numbers of the positions in the text:

(string-match
 "\\(qu\\)\\(ick\\)" "The quick fox jumped quickly.")
     ⇒ 4            ;^^^^^^^^^^
                     ;0123456789      

(match-beginning 1)               ; The beginning of the match
     ⇒ 4                         ;   with ‘qu’ is at index 4.

(match-beginning 2)               ; The beginning of the match
     ⇒ 6                         ;   with ‘ick’ is at index 6.

(match-end 1)                     ; The end of the match
     ⇒ 6                         ;   with ‘qu’ is at index 6.

(match-end 2)                     ; The end of the match
     ⇒ 9                         ;   with ‘ick’ is at index 9.

Here is another example. Before the form is evaluated, point is located at the beginning of the line. After evaluating the search form, point is located on the line between the space and the word ‘in’. The beginning of the entire match is at the 9th character of the buffer (‘T’), and the beginning of the match for the first subexpression is at the 13th character (‘c’).

(list
  (re-search-forward "The \\(cat \\)")
  (match-beginning 0)
  (match-beginning 1))
    ⇒ (t 9 13)

---------- Buffer: foo ----------
I read "The cat ∗in the hat comes back" twice.
        ^   ^
        9  13
---------- Buffer: foo ----------

(Note that in this case, the index returned is a buffer position; the first character of the buffer counts as 1.)

Function: match-end count

This function returns the position of the end of text matched by the last regular expression searched for. This function is otherwise similar to match-beginning.

1.5.2 Replacing the Text That Matched

Function: replace-match replacement &optional fixedcase literal

This function replaces the text matched by the last search with replacement.

If fixedcase is non-nil, then the case of the replacement text is not changed; otherwise, the replacement text is converted to a different case depending upon the capitalization of the text to be replaced. If the original text is all upper case, the replacement text is converted to upper case, except when all of the words in the original text are only one character long. In that event, the replacement text is capitalized. If all of the words in the original text are capitalized, then all of the words in the replacement text are capitalized.

If literal is non-nil, then replacement is inserted exactly as it is, the only alterations being case changes as needed. If it is nil (the default), then the character ‘\’ is treated specially. If a ‘\’ appears in replacement, then it must be part of one of the following sequences:

‘\&’

‘\&’ stands for the entire text being replaced.

‘\n’

‘\n’ stands for the nth subexpression in the original regexp. Subexpressions are those expressions grouped inside of ‘\(…\)’. n is a digit.

‘\\’

‘\\’ stands for a single ‘\’ in the replacement text.

replace-match leaves point at the end of the replacement text, and returns t.

1.5.3 Accessing the Entire Match Data

The functions match-data and store-match-data let you read or write the entire match data, all at once.

Function: match-data

This function returns a new list containing all the information on what text the last search matched. Element zero is the position of the beginning of the match for the whole expression; element one is the position of the end of the match for the expression. The next two elements are the positions of the beginning and end of the match for the first subexpression. In general, element corresponds to (match-beginning n); and element corresponds to (match-end n).

All the elements are markers or nil if matching was done on a buffer, and all are integers or nil if matching was done on a string with string-match. (In Emacs 18 and earlier versions, markers were used even for matching on a string, except in the case of the integer 0.)

As always, there must be no possibility of intervening searches between the call to a search function and the call to match-data that is intended to access the match-data for that search.

(match-data)
     ⇒  (#<marker at 9 in foo>
          #<marker at 17 in foo>
          #<marker at 13 in foo>
          #<marker at 17 in foo>)

Function: store-match-data match-list

This function sets the match data from the elements of match-list, which should be a list that was the value of a previous call to match-data.

If match-list refers to a buffer that doesn’t exist, you don’t get an error; that sets the match data in a meaningless but harmless way.

1.5.4 Saving and Restoring the Match Data

All asynchronous process functions (filters and sentinels) and functions that use recursive-edit should save and restore the match data if they do a search or if they let the user type arbitrary commands. Saving the match data is useful in other cases as well—whenever you want to access the match data resulting from an earlier search, notwithstanding another intervening search.

This example shows the problem that can arise if you fail to attend to this requirement:

(re-search-forward "The \\(cat \\)")
     ⇒ 48
(foo)                   ; Perhaps foo does
                        ;   more searching.
(match-end 0)
     ⇒ 61              ; Unexpected result---not 48!

In Emacs versions 19 and later, you can save and restore the match data with save-match-data:

Special Form: save-match-data body…

This special form executes body, saving and restoring the match data around it. This is useful if you wish to do a search without altering the match data that resulted from an earlier search.

You can use store-match-data together with match-data to imitate the effect of the special form save-match-data. This is useful for writing code that can run in Emacs 18. Here is how:

(let ((data (match-data)))
  (unwind-protect
      ...   ; May change the original match data.
    (store-match-data data)))

1.6 Standard Regular Expressions Used in Editing

Here are the regular expressions standardly used in editing:

Variable: page-delimiter

This is the regexp describing line-beginnings that separate pages. The default value is "^\014" (i.e., "^^L" or "^\C-l").

Variable: paragraph-separate

This is the regular expression for recognizing the beginning of a line that separates paragraphs. (If you change this, you may have to change paragraph-start also.) The default value is "^[ \t\f]*$", which is a line that consists entirely of spaces, tabs, and form feeds.

Variable: paragraph-start

This is the regular expression for recognizing the beginning of a line that starts or separates paragraphs. The default value is "^[ \t\n\f]", which matches a line starting with a space, tab, newline, or form feed.

Variable: sentence-end

This is the regular expression describing the end of a sentence. (All paragraph boundaries also end sentences, regardless.) The default value is:

"[.?!][]\"')}]*\\($\\|\t\\| \\)[ \t\n]*"

This means a period, question mark or exclamation mark, followed by a closing brace, followed by tabs, spaces or new lines.

For a detailed explanation of this regular expression, see Complex Regexp Example.

1.7 Searching and Case

By default, searches in Emacs ignore the case of the text they are searching through; if you specify searching for ‘FOO’, then ‘Foo’ or ‘foo’ is also considered a match. Regexps, and in particular character sets, are included: thus, ‘[aB]’ would match ‘a’ or ‘A’ or ‘b’ or ‘B’.

If you do not want this feature, set the variable case-fold-search to nil. Then all letters must match exactly, including case. This is a per-buffer-local variable; altering the variable affects only the current buffer. (@xref{Intro to Buffer-Local}.) Alternatively, you may change the value of default-case-fold-search, which is the default value of case-fold-search for buffers that do not override it.

User Option: case-replace

This variable determines whether query-replace should preserve case in replacements. If the variable is nil, then case need not be preserved.

User Option: case-fold-search

This buffer-local variable determines whether searches should ignore case. If the variable is nil they do not ignore case; otherwise they do ignore case.

Variable: default-case-fold-search

The value of this variable is the default value for case-fold-search in buffers that do not override it. This is the same as (default-value 'case-fold-search).

About This Document

This document was generated on January 16, 2023 using texi2html 5.0.

The buttons in the navigation panels have the following meaning:

where the Example assumes that the current position is at Subsubsection One-Two-Three of a document of the following structure:

• 1. Section One
  • 1.1 Subsection One-One
    • ...
  • 1.2 Subsection One-Two
    • 1.2.1 Subsubsection One-Two-One
    • 1.2.2 Subsubsection One-Two-Two
    • 1.2.3 Subsubsection One-Two-Three     <== Current Position
    • 1.2.4 Subsubsection One-Two-Four
  • 1.3 Subsection One-Three
    • ...
  • 1.4 Subsection One-Four

This document was generated on January 16, 2023 using texi2html 5.0.