Transform a string into a number : expr

### 2.4.11 Transform a string into a number : `expr`

Use `expr`, the parser with a string representing a number.

- For integers, enter the string representing the integer without leading 0 for basis 10, with prefix `0x` for basis 16, `0` for basis 8 or `0b` for basis 2. Input :
  `expr("123")`
  Output :
  `123`
  Input :
  `expr("0123")`
  Output :
  `83`
  Because : 1*82+2*8+3=83 Input :
  `expr("0x12f")`
  Output :
  `303`
  Because : 1*162+2*16+15=303
- For decimal numbers, use a string with a `.` or `e` inside.
  Input :
  `expr("123.4567")`
  Output :
  `123.4567`
  Input :
  `expr("123e-5")`
  Output :
  `0.00123`
- Note that `expr` more generally transforms a string into a command if the command exists.
  Input :
  `expr("a:=1")`
  Output :
  `1`
  Then, input :
  `a`
  Output :
  `1`