Ruby infixed math parser
Your parseChunk function is not really parsing an expression — it is also evaluating it.  Its naming style is also inconsistent with is_number?.  I think that eval_binary_expr would be a more appropriate name.

Augmenting the String class with an is_number? method is something to avoid doing.  That kind of patching increases the chances of breakage when your code coexists with other code in the same program.

In functions that are meant to be used by other code, calling exit is not an appropriate way to handle errors.  You should raise exceptions instead, so that the caller can handle the error in whatever way it chooses.

You appear to be working much too hard to parse the binary expression.  I think that this would accomplish more or less the same thing:

def eval_binary_expr(expr)
  l_operand, op, r_operand = expr.partition(%r{[/*+-]})

  if op.empty?
    raise ArgumentError, "Invalid operation or no operation in expression: #{expr}"
  end
  l_operand, r_operand = Float(l_operand), Float(r_operand)

  case op
    when '/' then l_operand / r_operand
    when '*' then l_operand * r_operand
    when '+' then l_operand + r_operand
    when '-' then l_operand - r_operand
  end
end


The solution above makes no attempt to handle negative numbers correctly.  As with your original solution, it fails when the first operand is negative.