Text Manipulator: String position movement

The task is to build to a text manipulator: A program that simulates a set of text manipulation commands.Given an input piece of text and a string of commands, output the mutated input text and cursor position.

Starting simple:

Commands

 h: move cursor one character to the left
 l: move cursor one character to the right
 r<c>: replace character under cursor with <c>

Repeating commands

# All commands can be repeated N times by prefixing them with a number.
# 
# [N]h: move cursor N characters to the left
# [N]l: move cursor N characters to the right
# [N]r<c>: replace N characters, starting from the cursor, with <c> and move the cursor

Examples

# We'll use Hello World as our input text for all cases:
# 
#  Input: hhlhllhlhhll
# Output: Hello World
#           _
#           2
# 
#  Input: rhllllllrw
# Output: hello world
#               _
#               6
# 
#  Input: rh6l9l4hrw
# Output: hello world
#               _
#               6
# 
#  Input: 9lrL7h2rL
# Output: HeLLo WorLd
#            _
#            3
# 
#  Input: 999999999999999999999999999lr0
# Output: Hello Worl0
#                   _
#                  10
# 
#  Input: 999rsom
# Output: sssssssssss
#                   _
#                  10

I have written the following piece of code, but getting an error:

class Editor():
    def __init__(self, text):
        self.text = text
        self.pos = 0

    def f(self, step):
        self.pos += int(step)

    def b(self, step):
        self.pos -= int(step)

    def r(self, char):
        s = list(self.text)
        s[self.pos] = char
        self.text = ''.join(s)

    def run(self, command):
        command = list(command)
        # if command not in ('F','B', 'R'):
        #
        while command:
            operation = command.pop(0).lower()
            if operation not in ('f','b','r'):
                raise ValueError('command not recognized.')
            method = getattr(self, operation)
            arg = command.pop(0)
            method(arg)

    def __str__(self):
        return self.text

# Normal run
text = 'abcdefghijklmn'
command = 'F2B1F5Rw'

ed = Editor(text)
ed.run(command)
print(ed)

I have used 'F' and 'B' in my code instead of 'h' and 'l', but the problem is I am missing a piece that allows me to define the optional 'N'. My code only works if there is a number defined after an operation. How can I fix the code above to meet all the requirements?

from Text Manipulator: String position movement