utf8substring -- extract part of a utf8 string

• Usage:

  utf8substring(s, i, n)

• Inputs:
  • s, a string,
  • i, an integer, starting index of substring
  • n, an integer, length of substring
• Outputs:
  • a string,

i1 : s = "π ≈ 3.14159";

i2 : utf8substring(s, 4, 4)

o2 = 3.14