Multiple prefix operators fail

[polluks]

>>> say \\1
+++ Error 35: Invalid expression
>>> say --1
+++ Error 35: Invalid expression
>>> say ++1
+++ Error 35: Invalid expression

[RossPatterson]

I'm surprised that this is proper Rexx, but you're right, it is. TRL2 doesn't have a grammar for expressions, but it says

Expressions consist of one or more terms (literal strings, symbols, function calls, or sub-expressions), and zero or more operators that denote operations to be carried out on terms. Most operators act on two terms, and there will be at least one of these dyadic operators between every pair ofterms. 17 There are also prefix (monadic) operators, that act on the term that is immediately to the right of the operator. There may be one or more prefix operators to the left of any term.
(pp. 24, emphasis mine)

ANSI does have a grammar, and it's very clear:

prefix_expression := ('+' | '-' | '') prefix_expression | term | Msg35.1 (6.3.2.112)

So TRL2 implies, and ANSI is explicit, that an unlimited number prefix operators can be combined, regardless of which operator is in use. So, for example, this should be legal:

True = (0=0)
False = \True
MinusOne = -\False` 
Say MinusOne   /* >>> "-1" */

Your fix is a good start, but it only allows sequences of the same operator, not of differing ones.

[polluks]

I see. ARexx fails MinusOne but Regina works!

[RossPatterson]

And bREXX for CMS fails, too, of course. I'll merge whatever the final fix is into that too.

[polluks]

This works

say -\0
say \(-0)
say \-0

however the last statement fails with Regina, confusing.
In the past I was using the interpreter of planet.sdf.org as reference but it's down.

[RossPatterson]

Here's one from VM/SP 5 (which doesn't support "\" as "not", but instead the EBCDIC x'5F' "logical not sign"):

test
     3 *-* True = (0=0)
       >L>   "0"
       >L>   "0"
       >O>   "1"
     4 *-* False = ¬True
       >V>   "1"
       >P>   "0"
     5 *-* MinusOne = -¬False
       >V>   "0"
       >P>   "1"
       >P>   "-1"
     6 *-* Say MinusOne   /* >>> "-1" */
       >V>   "-1"
-1
Ready;

Note that it is syntactically legal to apply a prefix operator to a variable, and for a runtime error to result due to an invalid input value:

test
     3 *-* MinusOne = -1
       >L>   "1"
       >P>   "-1"
     4 *-* Junk = ¬MinusOne
       >V>   "-1"
     4 +++ Junk = ¬MinusOne
Error 34 running TEST EXEC, line 4: Logical value not 0 or 1
Ready(20034);

[RossPatterson]

say \-0
however the last statement fails with Regina, confusing.

Regina 3.97 calls that a syntax error:

Error 35 running "C:\Ross\test.rexx", line 13: Invalid expression
Error 35.1: Invalid expression detected at ""

But I think it's a bug, based on the above.