Extracting whole words

Question

I have a large set of real-world text that I need to pull words out of to input into a spell checker. I'd like to extract as many meaningful words as possible without too much noise. I know there's plenty of regex ninjas around here, so hopefully someone can help me out.

Currently I'm extracting all alphabetical sequences with '[a-z]+'. This is an okay approximation, but it drags a lot of rubbish out with it.

Ideally I would like some regex (doesn't have to be pretty or efficient) that extracts all alphabetical sequences delimited by natural word separators (such as [/-_,.: ] etc.), and ignores any alphabetical sequences with illegal bounds.

However I'd also be happy to just be able to get all alphabetical sequences that ARE NOT adjacent to a number. So for instance 'pie21' would NOT extract 'pie', but 'http://foo.com' would extract ['http', 'foo', 'com'].

I tried lookahead and lookbehind assertions, but they were applied per-character (so for example re.findall('(?<!\d)[a-z]+(?!\d)', 'pie21') would return 'pi' when I want it to return nothing). I tried wrapping the alpha part as a term ((?:[a-z]+)) but it didn't help.

More detail: The data is an email database, so it's mostly plain English with normal numbers, but occasionally there's rubbish strings like GIHQ4NWL0S5SCGBDD40ZXE5IDP13TYNEA and AC7A21C0 that I'd like to ignore completely. I'm assuming any alphabetical sequence with a number in it is rubbish.

Answer 1

If you restrict yourself to ASCII letters, then use (with the re.I option set)

\b[a-z]+\b

\b is a word boundary anchor, matching only at the start and end of alphanumeric "words". So \b[a-z]+\b matches pie, but not pie21 or 21pie.

To also allow other non-ASCII letters, you can use something like this:

\b[^\W\d_]+\b

which also allows accented characters etc. You may need to set the re.UNICODE option, especially when using Python 2, in order to allow the \w shorthand to match non-ASCII letters.

[^\W\d_] as a negated character class allows any alphanumeric character except for digits and underscore.

Answer 2

Are you familiar with word boundaries? (\b). You can extract word's using the \b around the sequence and matching the alphabet within:

\b([a-zA-Z]+)\b

For instance, this will grab whole words but stop at tokens such as hyphens, periods, semi-colons, etc.

You can the \b sequence, and others, over at the python manual

EDIT Also, if you're looking to about a number following or preceding the match, you can use a negative look-ahead/behind:

(?!\d)   # negative look-ahead for numbers
(?<!\d)  # negative look-behind for numbers

Answer 3

What about:

import re
yourString="pie 42 http://foo.com GIHQ4NWL0S5SCGBDD40ZXE5IDP13TYNEA  pie42"
filter (lambda x:re.match("^[a-zA-Z]+$",x),[x for x in set(re.split("[\s:/,.:]",yourString))])

Note that:

• split explodes your string into potential candidates => returns a list of "potential words"
• set makes unicity filtering => transforms the list in set, thus removing entries appearing more than once. This step is not mandatory.
• filter reduces the number of candidates : takes a list, applies a test function to each element, and returns a list of the element succeeding the test. In our case, the test function is "anonymous"
• lambda : anonymous function, taking an item and checking if it's a word (upper or lower letters only)

EDIT : added some explanations