Why a variable can't start with a number but can ends with it? [duplicate]

Question

This question already has answers here:

Why can't variable names start with numbers? (24 answers)

Closed 8 years ago.

In many languages (C#, Javascript, CSS, and so on) I can't declare this variable:

123test

but I can declare this:

test123

What's the real cause of it?

Answer 1

It simplifies the parser. It can tell from the first character in a token whether it's an identifier or a number.

Also, the syntax for floating point can look like this:

123e45

This means 123x10⁴⁵. If identifiers could start with a number, this could be confused with a variable.

There are some languages that don't have this prohibition, Common Lisp for instance. It's rule is essentially that the token is a symbol unless it can be parsed as a number. Since it also allows the input radix to be customized, it has the property that whether a token is a number or symbol depends on the setting of a variable (it also has escaping mechanisms that allow you to force it one way or the other).

Answer 2

Because then a string of digits would be a valid identifier as well as a valid number.

int 17 = 497;
int 42 = 6 * 9;
String 1111 = "Totally text";

@skiphoppy`s answear.

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