Write a function that subtract two integer streams

You are given two streams 's_a' and 's_b' of digits. Each stream represents an integer 'a' and 'b' from its less significant digit to the most significant digit.
For example, integer 2048 is represented in the stream as 8, 4, 0, 2. 

Write a function that subtract two integers 'a - b' and returns the result as a string. You are not allowed to buffer entire 'a' or 'b' into a single string, i.e. you may access only a single digit per stream at time (imagine that 'a' and 'b' are stored in huge files). You may assume that 'a>=b'.

#include <stdio.h>

/* Subtracts two single digits, prints the reminder and returns 1(carry) if a<b */
int subtract(char c_a, char c_b)
{
  /* Convert the char to interger */
  int i_a = c_a - '0';
  int i_b = c_b - '0';
  int ret = 0;

  /* If a<b, increase 'a' by 10 and return 1(carry) to subtract it from 'a' of the next set */
  if (i_a < i_b) {
    i_a += 10;
    ret = 1;
  }
  
  printf ("%d", i_a - i_b);

  return ret;
}


int main(int agrc, char **argv)
{
  /* Get the two operand streams from input arguments */
  char *s_a = argv[1];
  char *s_b = argv[2];

  int carry_one = 0;
  
  /* Pass one char at a time from each input arguments to function subtract */
  while(*s_b != '\0'){
    /* Subtract carry_one from first operand before sending it to function subtract */
    carry_one = subtract(*s_a - carry_one, *s_b);
    s_a++;
    s_b++;
  }

  /* Print the remining digits from the first operand */
  while(*s_a != '\0'){
    printf("%c", *s_a - carry_one);
    s_a++;
    carry_one = 0;
  }
  printf("\n");
  
  return 0;
}

Output:

  $ ./str_subtract 00051 1005 | rev
  09999
  
  $ ./str_subtract 5 3 | rev
  2
  
  $ ./str_subtract 1234 1233 | rev
  1000

Python - Converting character string to HEXA string

ord(c): Given a string of length one, return an integer representing the Unicode code point of the character when the argument is a unicode object, or the value of the byte when the argument is an 8-bit string.

>>> ord("a")
97
>>> ord("\t")
9

hex(n): Convert an integer number to a hexadecimal string

>>> hex (97)
'0x61'
>>> hex (9)
'0x9'

We do not need first to characters in the hex output.

>>> hex (97)[2:]
'61'
>>> hex (9)[2:]
'9'

zfill(witdh): Return the numeric string left filled with zeros in a string of length width.

>>> hex (97)[2:].zfill (3)
'061'
>>> hex (9)[2:].zfill (2)
'09'

Put of together,

>>> s="\x08\t\x07khr\x00"
>>> "".join ([hex (ord(c))[2:].zfill(2) for c in s])
'0809076b687200'

Reference:
href=http://docs.python.org/library/functions.html

Perl - Converting character string to HEXA string

   $buf =~ s/(.)/sprintf("%0X",ord($1))/eg;

Substitution Operator Modifiers
Here is the list of all modifiers used with substitution operator

Modifier Description
     i   Makes the match case insensitive
     m   Specifies that if the string has newline or carriage
         return characters, the ^ and $ operators will now
         match against a newline boundary, instead of a
         string boundary
     o   Evaluates the expression only once
     s   Allows use of . to match a newline character
     x   Allows you to use white space in the expression
         for clarity
     g   Replaces all occurrences of the found expression
         with the replacement text
     e   Evaluates the replacement as if it were a Perl statement,
         and uses its return value as the replacement text

References:
http://icfun.blogspot.com/2009/05/perl-convert-character-string-into-hex.html
http://www.tutorialspoint.com/perl/perl_regular_expression.htm