Re: regexp font-lock highlighting

[Juri Linkov <juri@jurta.org>]

> Note that I removed the "\\(\\?:\\)?" since I find it distracting to
> put yet another face here.  If you believe that you _really_ need it
> you will have to reinsert it, but in that case you have to modify
> match-data cropping as well.  (I do have to modify match-data since
> redisplay wants some valid buffer positions for highlighting.)

I fixed shy group handling in this regexp by allowing ?: to be only
before `(' instead of before any of `(', `|' or `)'.  You may need to
adjust your code before submitting your final patch.

Also I tried to extend this regexp to highlight other special backslash
constructs like `\<', `\_<', `\w', `\sC', but soon discovered that
sometimes there are similar constructs in non-regexp strings that become
inappropriately highlighted.  Unless it's possible to detect whether
a given string is a regexp or not, I think the regexp highlighting
code should be split into multi-level font-lock variables
font-lock-regexp-keywords-1 and font-lock-regexp-keywords-2.
The first variable would highlight only `\\(?:', `\\|' and `\\)' constructs,
but the second - all other regexp constructs.  Everyone agreed to see
regexp constructs sometimes highlighted in wrong places could use
the second level variable.

> Finally, I would use three distinct font-lock faces for regexps:
>
> - One face for highlighting the "\\"s which by default should inherit
>   from `font-lock-string-face' with a dimmed foreground - I'm using
>   Green4 for strings and PaleGreen3 for the "\\"s.  Anyone who doesn't
    =====                      =====

I see everyone tends to change the default color of `font-lock-string-face'
(which is barely visible on light backgrounds) to a green color.
Currently on tty strings are fontified in green, so maybe this is one
of the reasons why people prefer green strings.  This indicates that
we should change the current "RosyBrown" default to one of the shades
of green.

>   like the highlighting could revert to `font-lock-string-face'.
>
> - One face for highlighting the "(", "|" and ")" in these expressions.
>   I find `bold' good here but again would leave it to the user whether
>   she wants to turn off highlighting this.  Moreover, such a face could
>   allow paren-highlighting to _never_ match a paren with that face with
>   a paren with another face.  Consequently, paren-matching could finally
>   provide more trustable information within regular expressions.

I added two new faces: for a backslash and regexp constructs after
a backslash.  The former inherits from escape-glyph to be highlighted
like a backslash in escape sequences, the latter is bold by default.
I think there is no need for a face for shy groups.  It is fontified
now in the same face as other regexp constructs (bold).

> - One face for highlighting the innermost grouping.  Basically,
>   `underline' is not bad here but appears a bit noisy in multiline
>   expressions or things like
>
>   (concat "\\("
>           some-string
>           "\\)")

You could add this face after implementing correct highlighting of
nested groups.

-- 
Juri Linkov
http://www.jurta.org/emacs/