How would I extract number out of a String after a character?

Convert string to char array, then check each character to see if it is a digit using Character.isDigit(char c)

See isDigit Documentation

OR

Use regex, with the split function.

See Java regex Documentation

See an Example Here

if you have to take into account other actions (+, -, :? *) and or brackets, split might not be the easiest way to go.

If you want to do a split (and the purpose is to obtain only digit characters), use regular expression that splits at any non-digit character (i.e. "[^\d]+"). Then you would get an array containing only digit characters.

If you are dealing with mathematic expressions, spliting a string into a char array would be OK because you will need to deal with the whole string (numbers & operands).