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What would be the best way to detect if a pattern corresponds to a string pattern ? An invalid string pattern p would lead to an error in StringMatchQ[string, p] for example.

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Have a look at System`Dump`validStringExpressionQ[]. – J. M.♦ Nov 20 '12 at 9:46
Yes that's the kind of built-in function I was looking for thanks. Maybe you could put the comment as an answer. – Faysal Aberkane Nov 20 '12 at 9:51

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up vote 6 down vote accepted

At Faysal's behest: there's the (undocumented) function System`Dump`validStringExpressionQ[]:

System`Dump`validStringExpressionQ["**"]
   True

System`Dump`validStringExpressionQ[_ ~~ 1]
   False
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A look at ??System`Dump`validStringExpressionQ is particularly enlightening... – J. M.♦ Nov 20 '12 at 10:01
Indeed so basically the pattern is passed to StringMatchQ in the general case and the function checks that no error occured. – Faysal Aberkane Nov 20 '12 at 10:31
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+1. A look at the implementation reveals some unpleasant behavior,e.g. stpatt = "**"; System`Dump`validStringExpressionQ[stpatt] returns False, while System`Dump`validStringExpressionQ[Evaluate[stpatt]] returns True. I would prefer to get True in both cases. – Leonid Shifrin Nov 20 '12 at 10:33
up vote 8 down vote 
StringPatternQ[_String | _StringExpression | _RegularExpression] = True;
StringPatternQ[_] = False;
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