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I need to check if the expression is a correct equation with only 1 variable x and without functions, that are not build into Mathematica. For example 

Sin[x]==0

is correct equation. And expressions 

Sin[x*a]==0, Sin[[x]==0, f[x]==0, Exp[x]=9

are not correct equations.

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    $\begingroup$ Are you testing these as strings or as expressions? Because one of them sticks out immediately being syntactically incorrect, so Mathematica won't have a problem checking whether it is a well-formed equation or not. $\endgroup$
    – István Zachar
    Dec 1, 2013 at 19:26
  • $\begingroup$ I'm testing these as strings. When expression is incorrect I will get message. The best variant for me is to get messages in other cases. $\endgroup$
    – user10893
    Dec 1, 2013 at 19:37

1 Answer 1

Reset to default
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How about this?

admissibleEqPat = 
  eq_Equal /; 
   DeleteDuplicates[
     Context /@ 
      DeleteCases[
       Cases[eq, _Symbol, Infinity, 
        Heads -> True], x]] === {"System`"};


MatchQ[x^2 + 3 x == 4 x^2/(x + 2), admissibleEqPat]
(* True *)

MatchQ[Sin[x] == 0, admissibleEqPat]
(* True *)

MatchQ[Sin[a x] == 0, admissibleEqPat]
(* False *)

MatchQ[f[x] == 0, admissibleEqPat]
(* False *)

I'm assuming that by built-in functions, you mean the standard built-in ones that are in the System` context.

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  • $\begingroup$ The problem is I need to add all functions, which are possible in my equations to admissibleSymbols. for example MatchQ[Sin[x] == 0, admissibleEqPat] (* False *) $\endgroup$
    – user10893
    Dec 1, 2013 at 19:53
  • $\begingroup$ @user10893 Check the update. $\endgroup$
    – Michael E2
    Dec 1, 2013 at 20:02

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