Determine whether input is a floating number

I am needing help creating multiple if-statements that can use string and character classes to determine whether the user input is a floating number. I know parsefloat can do this but we are not allowed to do this. I know the if-statements must be that I at most i can only have one decimal point, one e, 1 positive or negative number at the beginning or after the e. There must be a number before and after the decimal and thats all i can think of right now. Basically a class that does what parsefloat but with if statements. Any help would be awesome. Not sure on what to put in the if statements.

I have one if statement for( int i = 0; i