Question

I’m trying to match a specific pattern using the re module in python.
I wish to match a full sentence (More correctly I would say that they are alphanumeric string sequences separated by spaces and/or punctuation)

Eg.

• “This is a regular sentence.”
• “this is also valid”
• “so is This ONE”

I’m tried out of various combinations of regular expressions but I am unable to grasp the working of the patterns properly, with each expression giving me a different yet inexplicable result (I do admit I am a beginner, but still).

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I’m tried:

• “((\w+)(\s?))*”

  To the best of my knowledge this should match one or more alpha alphanumerics greedily followed by either one or no white-space character and then it should match this entire pattern greedily. This is not what it seems to do, so clearly I am wrong but I would like to know why. (I expected this to return the entire sentence as the result)
  The result I get for the first sample string mentioned above is [(‘sentence’, ‘sentence’, ”), (”, ”, ”), (”, ”, ”), (”, ”, ”)].

• “(\w+ ?)*”

  I’m not even sure how this one should work. The official documentation(python help(‘re’)) says that the ,+,? Match x or x (greedy) repetitions of the preceding RE.
  In such a case is simply space the preceding RE for ‘?’ or is ‘\w+ ‘ the preceding RE? And what will be the RE for the ‘‘ operator? The output I get with this is [‘sentence’].

• Others such as “(\w+\s?)+)” ; “((\w*)(\s??)) etc. which are basically variation of the same idea that the sentence is a set of alpha numerics followed by a single/finite number of white spaces and this pattern is repeated over and over.

Can someone tell me where I go wrong and why, and why the above expressions do not work the way I was expecting them to?

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P.S I eventually got “[ \w]+” to work for me but With this I cannot limit the number of white-space characters in continuation.

Answer 1

Your reasoning about the regex is correct, your problem is coming from using capturing groups with *. Here’s an alternative:

>>> s="This is a regular sentence."
>>> import re
>>> re.findall(r'\w+\s?', s)
['This ', 'is ', 'a ', 'regular ', 'sentence']

In this case it might make more sense for you to use \b in order to match word boundries.

>>> re.findall(r'\w+\b', s)
['This', 'is', 'a', 'regular', 'sentence']

Alternatively you can match the entire sentence via re.match and use re.group(0) to get the whole match:

>>> r = r"((\w+)(\s?))*"
>>> s = "This is a regular sentence."
>>> import re
>>> m = re.match(r, s)
>>> m.group(0)
'This is a regular sentence'