Bug introduced in 5 or earlier and persisting through 12.0
Anyone knows why
StringMatchQ["x", Except[{"1", "2"}]]
StringMatchQ["x", Except[{"*"}]]
StringMatchQ["x", {"1", "*"}] (* without Except *)
all work as expected, but
StringMatchQ["x", Except[{"1", "*"}]]
throws the following error:
StringExpression::invld: Element Except[{1,*}] is not a valid string
or pattern element in StringExpression[Except[{1,*}]]. >>
I don't think that StringMatchQ["x", Except[{"*"}]] works as expected as well as StringMatchQ["x", Except["*"]]:
StringMatchQ["x", Except[{"*"}]]
StringMatchQ["x", Except["*"]]
True True
The string pattern "*" is an abbreviated string pattern consisted from the only metacharacter * which corresponds to zero or more characters according to the first point under the "Details and Options" section of StringMatchQ. So by the definition the pattern Except["*"] shouldn't give a match at all because it means negation of literally everything. Also for matching verbatim "*" one must escape this metacharacter with double backslash "\\*" (the reference is the same). So current behavior is a bug.
Note also that escaping with single backslash is meant for Mathematica's internal metacharacters like < and > (and obviously for more common n, t and r etc. but with another counting by StringLength) as described in this answer by John Fultz:
The modern Mathematica notebook format (introduced in 1996) was always made to be interpreted properly as a Mathematica expression should you call
Get[]on it from the kernel. So this syntax was standardized, and is still used today. Now, the kernel simply ignores the\<\>delimiters as you can see below:In[1]:== StringLength["\<x\>"]Out[1]== 1
The following demonstrates a possible bug (for the string "\*" length must be 2 because * isn't an internal metacharacter of Mathematica and it needs not be escaped inside of an ordinary string):
StringLength["*"]
StringLength["\*"]
1 1
StringLength["\x"]
Syntax::stresc: Unknown string escape \x.
2
The following is a set of examples of the expected behavior (observed with version 10.4.1):
StringMatchQ["", "*"]
StringMatchQ["", Except["*"]]
True False
StringMatchQ["xy", {"*"}]
StringMatchQ["xy", "*"]
True True
StringMatchQ["*", Except[{"*"}]]
StringMatchQ["*", Except["*"]]
StringMatchQ["xy", Except["*"]]
False False False
StringMatchQ["x", "@"]
True
StringMatchQ["x", {"\\*"}]
StringMatchQ["x", "\\*"]
StringMatchQ["*", "\\*"]
False False True
The following is a set of examples of wrong behavior of Except with metacharacters:
StringMatchQ["x", Except[{"\\*"}]] (* metacharacter is correctly escaped *)
StringMatchQ["x", Except["\\*"]] (* metacharacter is correctly escaped *)
StringExpression::invld: Element Except[{\*}] is not a valid string or pattern element in Except[{\*}]. >>
StringMatchQ["x", Except[{"\\*"}]]StringExpression::invld: Element Except[\*] is not a valid string or pattern element in Except[\*]. >>
StringMatchQ["x", Except["\\*"]]
StringMatchQ["x", Except["\*"]] (* metacharacter is wrongly escaped *)
StringMatchQ["*", Except["\*"]] (* metacharacter is wrongly escaped *)
True True
StringMatchQ["x", Except["@"]]
StringExpression::invld: Element Except[@] is not a valid string or pattern element in Except[@]. >>
StringMatchQ["x", Except["@"]]
And here is an example of wrong (but consistent) behavior of StringMatchQ both with Except and without it:
StringMatchQ["\*", Except["\*"]] (* metacharacter is wrongly escaped *)
StringMatchQ["\*", "\*"] (* metacharacter is wrongly escaped *)
False True
In addition:
StringMatchQ["*", Except["\*"]]
True
I've found an old MathGroup discussion on this topic from where I learned that StringCases and StringPosition don't support abbreviated string patterns, and that Verbatim forces StringMatchQ to match two strings literally: StringMatchQ["\\*", Verbatim["\\*"]] returns True.
