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This is the same issue as the compiled
two argument [switch].

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That is, 1899962.

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I believe the 2nd variant (eval version) is what should be fixed.  The extended info from wrong#args is:

wrong # args: should be "regexp ?switches? exp string ?matchVar? ?subMatchVar subMatchVar ...?"

This indicates 2 known required args, and 2 args are given to regexp.  Why try to find errors when we can otherwise assume a regexp+2args == [regexp $pat $str] at all times?

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I agree that is the most sensible
way to define the behavior of this
command.

The snag is that the implementation
over many years *and the documentation*
disagree, saying that anything that
starts with "-" gets an attempt at
interpretation as a switch.

This is stupid, and it foils many
cases that could otherwise be compiled,
so I agree we should change it like
we changed [switch].

Just be ready for the same flood of
complaints we got about the [switch]
change.

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Be aware of the test regexp-20.2

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dkf points out the roadblock case:

% regexp -about foo
0 {}
% set e -about
-about
% set s foo
foo
% regexp $e $s
0

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This is tclguy's turf, I believe (and hope)

Still the same in 8.6, 9.0 and 9.1. TclCompileRegexpCmd() can only check a literal RE for a leading "-"; an RE from a variable went straight to INST_REGEXP, which does not parse switches.

I disagree with changing regexp to match the compiled behaviour: compilation is only an optimization, and the compiled code should always behave like the interpreted command. Proposed fix in branch regexp-compile-runtime-switch: when the RE is not known at compile time and there is no --, the compiled code checks at runtime whether it starts with "-", and if so calls regexp itself. So compiled and interpreted regexp now behave the same, including -about, and ordinary REs still use INST_REGEXP. Tests regexpComp-25.1 to 25.7.

With -- the RE cannot be a switch, so the compiled code needs no runtime check: regexp -- $re $string is the safest and fastest way to match a variable RE. Much code in Tcl's library is already written this way.