Given a string of positive number ranging from 0 to 9, the task is to check whether the number is valid pin code or not, by using Python Regular Expression.

Sure, here is a step by step guide on how to validate a pin code using Python Regular Expression:

1. Import the re module: This is Python's built-in module for regular expressions.

import re

2. Define the pattern for a valid pin code: A valid pin code is a string of exactly 6 digits. We can represent this pattern with the regular expression '^\d{6}$'. Here, ^ indicates the start of the string, \d represents any digit, {6} means exactly 6 repetitions of the previous character (any digit), and $ indicates the end of the string.

pattern = '^\d{6}$'

3. Compile the pattern into a Pattern object: This step is not necessary, but it improves performance if you're going to use the same pattern multiple times.

pin_code_pattern = re.compile(pattern)

4. Define a function that checks whether a string is a valid pin code: This function takes a string as input, tries to match it against the pin code pattern, and returns True if it's a match and False otherwise.

def is_valid_pin_code(pin_code):
    if pin_code_pattern.match(pin_code):
        return True
    else:
        return False

5. Test the function with some example pin codes:

print(is_valid_pin_code('123456'))  # True
print(is_valid_pin_code('12345'))   # False
print(is_valid_pin_code('1234567')) # False
print(is_valid_pin_code('12345a'))  # False

This program will correctly identify '123456' as a valid pin code, and '12345', '1234567', and '12345a' as invalid pin codes.