Question

So, I’ve seen this type thing on several occasions, but I’ve never been able to find out anything about it. Can someone please explain what these “r’ foo(*)'” things are and link me to the python docs for them? I think they have to do with finding key words in strings. Am I right?

(r'why (.*) i (.*)\?', 
( "You%1%2?", 
"Perhaps you only think you%1%2")), 

(r'why (.*) you(.*)\?', 
( "Why%1 you%2?", 
"%2 I%1", 
"Are you sure I%2?")), 

(r'why (.*)\?', 
( "I cannot tell you why%1.", 
"Why do you think %1?" )), 

This particular segment of code is from the nltk.chat.zen module.

Answer 1

Those appear to be regular expressions. Regular expressions let you search for patterns in strings in a fairly powerful and sophisticated way. They can be a bit cryptic though. These appear to be the basis of some kind of primitive chatterbot.

>>> import re
>>> regex, phrases = (r'why (.*) i (.*)\?', 
... ( "You %s %s?", 
... "Perhaps you only think you %s %s"))
>>> phrase1, phrase2 = phrases
>>> groups = re.search(regex, 'why am i dumb?')
>>> phrase1 % (groups.group(1), groups.group(2))
'You am dumb?'
>>> phrase2 % (groups.group(1), groups.group(2))
'Perhaps you only think you am dumb'

Ah, and perhaps you were specifically wondering what r'blahblah' means. Well, as others have explained, the r just makes it a raw string — Python doesn’t do certain kinds of processing on it, which makes REs easier to read.

---

Here’s something closer to what I think the original intent was for these strings. They don’t seem to be using standard Python group substitution syntax, so I assume they must be using a custom substitution function for this — perhaps one sophisticated enough to handle converting the verb form! You can see the basic idea here.

>>> import re
>>> regex, phrases = (r'why (.*) i (.*)\?',
... ( r'You \g<1> \g<2>?',
...   r'Perhaps you only think you \g<1> \g<2>.'))
>>> phrase1, phrase2 = phrases
>>> re.sub(regex, phrase1, 'why am i dumb?')
'You am dumb?'
>>> re.sub(regex, phrase2, 'why am i dumb?')
'Perhaps you only think you am dumb.'