Font locking (syntax highlighting) doesn't work with keywords starting with a number

[BrunoBonacci]

Expected behavior

The following are both valid keywords, however, the font locking doesn't work on the second one

:foo
:1foo 

Actual behavior

Keywords starting with a number are not colorised as keywords

Steps to reproduce the problem

see example above

Environment & Version information

clojure-mode version

Include here the version string displayed by M-x clojure-mode-display-version. Here's an example:

clojure-mode (version 20201126.1558)
clojure-mode (version 5.13.0-snapshot)

Emacs version

GNU Emacs 27.1

Operating system

Mac OS 11.1

[bbatsov]

Probably the regexp for font-locking keywords needs to be tweaked.

[yuhan0]

I don't think this is a bug, such keywords are not syntactically correct even though the reader currently accepts them. https://stackoverflow.com/a/39193032

[bbatsov]

Yeah, that's technically true, but on the other hand I also know this is never going to be fixed, as it will break backwards compatibility. That's why probably it makes sense to acknowlege this weird case as valid.

[BrunoBonacci]

Maybe the bug is in the clojuredocs.org description as the official doc doesn't mention this constraint
https://clojure.org/reference/data_structures#Keywords

Keywords are symbolic tokens that express a concept/value by themselves like: :blue, :green, :slow, :fast.
From this point of view it makes sense that I should be able to express things like: :30sec, :1h, :1mb, :3XL if they make sense for someone's program.

I know that there are other ways to express such values, but the point is that the language should allow you to express it like this is you wish so.

The same consideration could be valid for the symbols themselves. The only reason why symbols have to start with a non-numerical character is to avoid ambiguity with the various numerical qualifiers, such as: 3e5, 25N, and 1.3M. Without this constraint, it would be impossible for the parser to understand whether they are to be considered symbols or numerical values. But such a problem is not present with keywords as they are unambiguously starting with :.
Anyway, that's my opinion.

I had a look at the source, and the keyword pattern reuses the definition of the symbol, which makes the fix slightly more complicated.

[OknoLombarda]

This happens because clojure-font-lock-keywords reuses regexp for Clojure symbol when looking for keywords, which disallows numbers as the first character

clojure-mode/clojure-mode.el

Line 977 in ad322e9

(,(concat "\\(:\\{1,2\\}\\)\\(" clojure--sym-regexp "?\\)\\(/\\)\\(" clojure--sym-regexp "\\)")

It can be fixed by introducing a separate regexp specifically for Clojure keyword symbol

clojure-mode/clojure-mode.el

Lines 779 to 782 in ad322e9

	(defconst clojure--sym-regexp
	(concat "[^" clojure--sym-forbidden-1st-chars "][^" clojure--sym-forbidden-rest-chars "]*")
	"A regexp matching a Clojure symbol or namespace alias.
	Matches the rule `clojure--sym-forbidden-1st-chars' followed by

 Matches the rule `clojure--sym-forbidden-1st-chars' followed by
+any number of matches of `clojure--sym-forbidden-rest-chars'.")
+  (defconst clojure--keyword-sym-forbidden-1st-chars
+    (concat clojure--sym-forbidden-rest-chars ":'")
+    "A list of chars that a Clojure keyword symbol cannot start with.")
+  (defconst clojure--keyword-sym-regexp
+    (concat "[^" clojure--keyword-sym-forbidden-1st-chars "]"
+            "[^" clojure--sym-forbidden-rest-chars "]*")
+    "A regexp matching a Clojure keyword name or keyword namespace.
+Matches the rule `clojure--keyword-sym-forbidden-1st-chars' followed by
 any number of matches of `clojure--sym-forbidden-rest-chars'."))

I tried to copy the way the symbol regexp is written (save the formatting), though it looks a little too verbose to me. If it looks okay, I'll send a PR

[bbatsov]

Looks reasonable to me.