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23.6.2 Search-based Fontification

The variable which directly controls search-based fontification is font-lock-keywords, which is typically specified via the keywords element in font-lock-defaults.

— Variable: font-lock-keywords

The value of this variable is a list of the keywords to highlight. Lisp programs should not set this variable directly. Normally, the value is automatically set by Font Lock mode, using the keywords element in font-lock-defaults. The value can also be altered using the functions font-lock-add-keywords and font-lock-remove-keywords (see Customizing Keywords).

Each element of font-lock-keywords specifies how to find certain cases of text, and how to highlight those cases. Font Lock mode processes the elements of font-lock-keywords one by one, and for each element, it finds and handles all matches. Ordinarily, once part of the text has been fontified already, this cannot be overridden by a subsequent match in the same text; but you can specify different behavior using the override element of a subexp-highlighter.

Each element of font-lock-keywords should have one of these forms:

regexp
Highlight all matches for regexp using font-lock-keyword-face. For example,
          ;; Highlight occurrences of the word ‘foo
          ;; using font-lock-keyword-face.
          "\\<foo\\>"

Be careful when composing these regular expressions; a poorly written pattern can dramatically slow things down! The function regexp-opt (see Regexp Functions) is useful for calculating optimal regular expressions to match several keywords.

function
Find text by calling function, and highlight the matches it finds using font-lock-keyword-face.

When function is called, it receives one argument, the limit of the search; it should begin searching at point, and not search beyond the limit. It should return non-nil if it succeeds, and set the match data to describe the match that was found. Returning nil indicates failure of the search.

Fontification will call function repeatedly with the same limit, and with point where the previous invocation left it, until function fails. On failure, function need not reset point in any particular way.

(matcher . subexp)
In this kind of element, matcher is either a regular expression or a function, as described above. The cdr, subexp, specifies which subexpression of matcher should be highlighted (instead of the entire text that matcher matched).
          ;; Highlight the ‘bar’ in each occurrence of ‘fubar’,
          ;; using font-lock-keyword-face.
          ("fu\\(bar\\)" . 1)

If you use regexp-opt to produce the regular expression matcher, you can use regexp-opt-depth (see Regexp Functions) to calculate the value for subexp.

(matcher . facespec)
In this kind of element, facespec is an expression whose value specifies the face to use for highlighting. In the simplest case, facespec is a Lisp variable (a symbol) whose value is a face name.
          ;; Highlight occurrences of ‘fubar’,
          ;; using the face which is the value of fubar-face.
          ("fubar" . fubar-face)

However, facespec can also evaluate to a list of this form:

          (face face prop1 val1 prop2 val2...)

to specify the face face and various additional text properties to put on the text that matches. If you do this, be sure to add the other text property names that you set in this way to the value of font-lock-extra-managed-props so that the properties will also be cleared out when they are no longer appropriate. Alternatively, you can set the variable font-lock-unfontify-region-function to a function that clears these properties. See Other Font Lock Variables.

(matcher . subexp-highlighter)
In this kind of element, subexp-highlighter is a list which specifies how to highlight matches found by matcher. It has the form:
          (subexp facespec [override [laxmatch]])

The car, subexp, is an integer specifying which subexpression of the match to fontify (0 means the entire matching text). The second subelement, facespec, is an expression whose value specifies the face, as described above.

The last two values in subexp-highlighter, override and laxmatch, are optional flags. If override is t, this element can override existing fontification made by previous elements of font-lock-keywords. If it is keep, then each character is fontified if it has not been fontified already by some other element. If it is prepend, the face specified by facespec is added to the beginning of the font-lock-face property. If it is append, the face is added to the end of the font-lock-face property.

If laxmatch is non-nil, it means there should be no error if there is no subexpression numbered subexp in matcher. Obviously, fontification of the subexpression numbered subexp will not occur. However, fontification of other subexpressions (and other regexps) will continue. If laxmatch is nil, and the specified subexpression is missing, then an error is signaled which terminates search-based fontification.

Here are some examples of elements of this kind, and what they do:

          ;; Highlight occurrences of either ‘foo’ or ‘bar’, using
          ;; foo-bar-face, even if they have already been highlighted.
          ;; foo-bar-face should be a variable whose value is a face.
          ("foo\\|bar" 0 foo-bar-face t)
          
          ;; Highlight the first subexpression within each occurrence
          ;; that the function fubar-match finds,
          ;; using the face which is the value of fubar-face.
          (fubar-match 1 fubar-face)

(matcher . anchored-highlighter)
In this kind of element, anchored-highlighter specifies how to highlight text that follows a match found by matcher. So a match found by matcher acts as the anchor for further searches specified by anchored-highlighter. anchored-highlighter is a list of the following form:
          (anchored-matcher pre-form post-form
                                  subexp-highlighters...)

Here, anchored-matcher, like matcher, is either a regular expression or a function. After a match of matcher is found, point is at the end of the match. Now, Font Lock evaluates the form pre-form. Then it searches for matches of anchored-matcher and uses subexp-highlighters to highlight these. A subexp-highlighter is as described above. Finally, Font Lock evaluates post-form.

The forms pre-form and post-form can be used to initialize before, and cleanup after, anchored-matcher is used. Typically, pre-form is used to move point to some position relative to the match of matcher, before starting with anchored-matcher. post-form might be used to move back, before resuming with matcher.

After Font Lock evaluates pre-form, it does not search for anchored-matcher beyond the end of the line. However, if pre-form returns a buffer position that is greater than the position of point after pre-form is evaluated, then the position returned by pre-form is used as the limit of the search instead. It is generally a bad idea to return a position greater than the end of the line; in other words, the anchored-matcher search should not span lines.

For example,

          ;; Highlight occurrences of the word ‘item’ following
          ;; an occurrence of the word ‘anchor’ (on the same line)
          ;; in the value of item-face.
          ("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))

Here, pre-form and post-form are nil. Therefore searching for ‘item’ starts at the end of the match of ‘anchor’, and searching for subsequent instances of ‘anchor’ resumes from where searching for ‘item’ concluded.

(matcher highlighters...)
This sort of element specifies several highlighter lists for a single matcher. A highlighter list can be of the type subexp-highlighter or anchored-highlighter as described above.

For example,

          ;; Highlight occurrences of the word ‘anchor’ in the value
          ;; of anchor-face, and subsequent occurrences of the word
          ;; item’ (on the same line) in the value of item-face.
          ("\\<anchor\\>" (0 anchor-face)
                          ("\\<item\\>" nil nil (0 item-face)))

(eval . form)
Here form is an expression to be evaluated the first time this value of font-lock-keywords is used in a buffer. Its value should have one of the forms described in this table.

Warning: Do not design an element of font-lock-keywords to match text which spans lines; this does not work reliably. For details, see See Multiline Font Lock.

You can use case-fold in font-lock-defaults to specify the value of font-lock-keywords-case-fold-search which says whether search-based fontification should be case-insensitive.

— Variable: font-lock-keywords-case-fold-search

Non-nil means that regular expression matching for the sake of font-lock-keywords should be case-insensitive.