bug#14481: 24.3.50; Highlighting escape sequences

[Dmitry Gutov <dgutov@yandex.ru>]

On 27.05.2013 18:16, Drew Adams wrote:
> But consider separating out the escape syntax for the various regexp
> parts - e.g., the parts that match more than a single escape char (for
> octal etc.).  Using separate regexps (which can still be combined) for
> the various parts lets you choose whether to highlight each char group
> (e.g., highlight only hex escapes).

I can't imagine why someone would want to highlight hex escapes, yet 
keep octals and others unhighlighted. If we're going to support some 
languages that only support a few escape sequences, then modifying this 
regexp might be the way to go, yes, although writing separate, smaller 
specialized regexp(s) might be better.

> FWIW, library highlight-chars.el lets you selectively highlight sets of
> chars.  This works together with font-lock: you can choose whether
> other font-lock highlighting overrides, is overridden by, or is merged
> with this highlighting (applied after or before it).
>
> You can specify chars to highlight in various ways: (1) individually
> (any in a given string), (2) using ranges, (3) using character classes
> (e.g. [:digit:]), and (4) using character sets (e.g. `iso-8859-1' or
> `lao').  You can also specify sets of chars to be excluded from such
> highlighting - IOW, specify a set by subtraction as well as by
> addition.

That's nice, but we really need regexp here. And there's no point 
relying on third-party library, the code is small as it is.