Limiting Python input strings to certain characters and lengths

Question

I just started learning my first real programming language  Python  I d like to know how to constrain user input in a raw input to certain characters and to a certain length  For example  I d like to show an error message if the user inputs a string that contains anything except the letters a-z  and I d like to show one of the user inputs more than 15 characters   The first one seems like something I could do with regular expressions  which I know a little of because I ve used them in Javascript things  but I m not sure how to use them in Python  The second one  I m not sure how to approach it  Can anyone help

User · Answer

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

User · Answer

if any    i gt  z  or i lt  a  for i in raw input        print  Error  Contains illegal characters  elif len raw input  gt 15      print  Very long string

User · Answer

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   gt  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   gt  15      print  Error  Only 15 characters allowed       sys exit    print  Your input was    input str

User · Answer

We can use assert here    def  input inp str str       try          assert len inp str  lt  15 print  More than 15 characters present           assert all  a  lt  i lt   z  for i in inp str  print  Characters other than  a - z  are found           return inp str     except Exception as e          pass      input  abcd    abcd  input  abc d    Characters other than  a - z  are found  input  abcdefghijklmnopqrst    More than 15 characters present

[python] Limiting Python input strings to certain characters and lengths

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