simple string problem

Split the string into tokens as explained in Solution 1.

Initialize an integer accumulator as 0, and a sign flag as positive.

Scan the list to identify the tokens (ignore the last one):

- every even token is either a number or a variable name,

- every odd token is either a + or an = .

To tell a number from a variable name, use sscanf("%d", ...) and check that it decodes one value. If number, accumulate it with its sign; if variable, save the sign.

When you meet the = token, change the sign flag to negative.

As you go analysing the string, construct another string by copying all tokens verbatim, except for the variable name, which you replace by "%d". (Alternatively, replace the variable name by %d in the original string.)

Format the result string by means of printf, passing the contructed string and the accumulator value taken with the sign of the variable, negated.

There you are. About a dozen lines of code. Totally unsafe to misformed input strings.

For one, this is not a 'simple string' problem, you want a Lexical Analyzer. There are libraries that solve the problem of tokenizing a text and structuring it in a sensible way, given set of grammatical rules. See the various references provided on the site linked above.