Limiting Python input strings to certain characters and lengths

Question 1: Restrict to certain characters

You are right, this is easy to solve with regular expressions:

import re

input_str = raw_input("Please provide some info: ")
if not re.match("^[a-z]*$", input_str):
    print "Error! Only letters a-z allowed!"
    sys.exit()

Question 2: Restrict to certain length

As Tim mentioned correctly, you can do this by adapting the regular expression in the first example to only allow a certain number of letters. You can also manually check the length like this:

input_str = raw_input("Please provide some info: ")
if len(input_str) > 15:
    print "Error! Only 15 characters allowed!"
    sys.exit()

Or both in one:

import re

input_str = raw_input("Please provide some info: ")
if not re.match("^[a-z]*$", input_str):
    print "Error! Only letters a-z allowed!"
    sys.exit()
elif len(input_str) > 15:
    print "Error! Only 15 characters allowed!"
    sys.exit()

print "Your input was:", input_str

We can use assert here.

def custom_input(inp_str: str):
    try:
        assert len(inp_str) <= 15, print("More than 15 characters present")
        assert all("a" <= i <= "z" for i in inp_str), print(
            'Characters other than "a"-"z" are found'
        )
        return inp_str
    except Exception as e:
        pass

---

custom_input('abcd')
#abcd
custom_input('abc d')
#Characters other than "a"-"z" are found
custom_input('abcdefghijklmnopqrst')
#More than 15 characters present

---

You can build a wrapper around input function.

def input_wrapper(input_func):
    def wrapper(*args, **kwargs):
        inp = input_func(*args, **kwargs)
        if len(inp) > 15:
            raise ValueError("Input length longer than 15")
        elif not inp.isalpha():
            raise ValueError("Non-alphabets found")
        return inp
    return wrapper

custom_input = input_wrapper(input)

Regexes can also limit the number of characters.

r = re.compile("^[a-z]{1,15}$")

gives you a regex that only matches if the input is entirely lowercase ASCII letters and 1 to 15 characters long.