Transform a string into a number : expr

### 5.4.11 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