How do I use regular expressions in Python with placeholder text?

Question

I am doing a project in Python where I require a user to input text. If the text matches a format supported by the program, it will output a response that includes a user's key word (it is a simple chat bot). The format is stored in a text file as a user input format and an answer format.

For example, the text file looks like this, with user input on the left and output on the right:

my name is <-name> | Hi there, <-name>

So if the user writes my name is johnny, I want the program to know that johnny is the <-name> variable, and then to print the response Hi there, johnny.

Some prodding me in the right direction would be great! I have never used regular expressions before and I read an article on how to use them, but unfortunately it didn't really help me since it mainly went over how to match specific words.

Answer 1

Here's an example:

import re

io = [
    ('my name is (?P<name>\w+)', 'Hi there, {name}'),
]

string = input('> ')
for regex, output in io:
    match = re.match(regex, string)
    if match:
        print(output.format(**match.groupdict()))
        break

I'll take you through it:

---

'my name is (?P<name>\w+)'

(?P<name>...) stores the following part (\w+) under the name name in the match object which we're going to use later on.

---

match = re.match(regex, string)

This looks for the regex in the input given. Note that re.match only matches at the beginning of the input, if you don't want that restriction use re.search here instead.

---

If it matches:

output.format(**match.groupdict())

match.groupdict returns a dictionary of keys defined by (?P<name>...) and their associated matched values. ** passes those key/values to .format, in this case Python will translate it to output.format(name='matchedname').

---

To construct the io dictionary from a file do something like this:

io = []
with open('input.txt') as file_:
    for line in file:
        key, value = line.rsplit(' | ', 1)
        io.append(tuple(key, value))