Matching math expression with regular expression?

Question

For example, these are valid math expressions:

a * b + c
-a * (b / 1.50)
(apple + (-0.5)) * (boy - 1)

And these are invalid math expression

Answer 1

Use a pushdown automaton for matching paranthesis http://en.wikipedia.org/wiki/Pushdown_automaton (or just a stack ;-) )

Details for the stack solution:

while (chr available)
    if chr == '(' then
      push '('
    else
      if chr == ')' then
        if stack.elements == 0 then
          print('too many or misplaced )')
          exit
        else
          pop //from stack
end while
if (stack.elements != 0)
  print('too many or misplaced(')

Even simple: just keep a counter instead of stack.

Answer 2

For parenthesis matching, and implementing other expression validation rules, it is probably easiest to write your own little parser. Regular expressions are no good in this kind of situation.