1. Accessing files with NLTK Regular Expressions

2. Accessing additional files
Python has tools for accessing files from the local directories and also for obtaining files from the web.

3. Python module for web access
urllib2
Note – this is for Python 2.x, not Python 3
Python 3 splits the urllib2 materials over several modules
import urllib2
urllib2.urlopen(url [,data][, timeout])
Establish a link with the server identified in the url and send either a GET or POST request to retrieve the page.
The optional data field provides data to send to the server as part of the request. If the data field is present, the HTTP request used is POST instead of GET
Use to fetch content that is behind a form, perhaps a login page
If used, the data must be encoded properly for including in an HTTP request. See
timeout defines time in seconds to be used for blocking operations such as the connection attempt. If it is not provided, the system wide default value is used.

4. URL fetch and use urlopen returns a file-like object with methods:
Same as for files: read(), readline(), readlines(), fileno(), close()
New for this class:
info() – returns meta information about the document at the URL
getcode() – returns the HTTP status code sent with the response (ex: 200, 404)
geturl() – returns the URL of the page, which may be different from the URL requested if the server redirected the request

5. Reminder, file access file.close() file.fileno() file.read([size])
File no longer available
file.fileno()
returns the file descriptor, not usually needed.
file.read([size])
read at most size bytes. If size not specified, read to end of file.
file.readline([size])
read one line. If size provided, read that many bytes. Empty string returned if EOF encountered immediately
file.readlines([sizehint])
return a list of lines. If sizehint present, return approximately that number of lines, possibly rounding to fill a buffer.
Where file is the internal name of the file object

6. URL info
info() provides the header information that http returns when the HEAD request is used.
ex:
>>> print mypage.info()
Date: Mon, 12 Sep :23:44 GMT
Server: Apache/ (Unix)
Last-Modified: Tue, 02 Sep :12:03 GMT
ETag: "2f0d4-215f-48bdac23"
Accept-Ranges: bytes
Content-Length: 8543
Connection: close
Content-Type: text/html

7. URL status and code >>> print mypage.getcode() 200
>>> print mypage.geturl()

8. Messy HTML HTML is not always perfect.
Browsers may be forgiving.
Human and computerized html generators make mistakes.
Tools for dealing with imperfect html include Beautiful Soup.
Beautiful Soup parses anything you give it, and does the tree traversal stuff for you. You can tell it "Find all the links", or "Find all the links of class externalLink", or "Find all the links whose urls match "foo.com", or "Find the table heading that's got bold text, then give me that text."

9. Exceptions: How to Deal with Error Situations
number = 0
while not 1 <= number <= 10:
try:
number= int(raw_input('Enter number from 1 to 10: '))
if not 1 <= number <= 10:
print 'Your number must be from 1 to 10:'
except ValueError:
print 'That is not a valid integer.'
Here: recognize an error condition and deal with it
If the named error occurs, the “except” clause is executed and the loop is terminated.
book slide

10. Checking for failed url fetch
import urllib2
url = raw_input("Enter the URL of the page to fetch: ")
try:
linecount=0
page=urllib2.urlopen(url)
result = page.getcode()
if result == 200:
for line in page:
print line
linecount+=1
print page.info()
print page.getcode()
print "Page contains ",linecount," lines."
except:
print "\nBad URL: ", url
The except clause is triggered by any error in the try

11. The NLP pipeline

12. Character encoding ASCII, Unicode
American Standard Code for Information Interchange
Everything stored in the computer must be expressed as a bit pattern.
For numbers, easy – convert to binary
For integers, direct conversion
For real numbers, floating point
somewhat arbitrary choice of how to represent where the decimal point is, how much precision for the whole number part, how much for the exponent.
For non-numeric characters, some arbitrary choice of what bit pattern to assign to each character

13. Coding considerations
If the numeric interpretation of the bit string assigned to one character is less than that for another character, the first will sort to an earlier position.
Thus, assign the codes in the sort order desired.
Clearly, A before B
A before or after a?
8 before or after A?
* before or after A, 8?
Once the choices are made and the code is constructed, sort order is determined. Any need to change will have to be dealt with in individual applications

14. Representing the bit patterns
All the encodings can be represented as numeric values. Example ASCII code for “K” – two bytes:
Decimal 75
familiar, but not really convenient for representing bits.
Hexadecimal 4B
one character for each four bits.
Octal (_ )
one character for each three bits, from the right

15. The ASCII code

16. Limitations of ASCII
Original ASCII used only 7 of the available 8 bits
last bit kept for parity checking
Limited to the number of characters that can be represented.
Extended – use the 8th bit
There are several variations
See

17. Extended ASCII Hex 80 to FF
Source:

18. Unicode ASCII is just one encoding example
ASCII, even extended, does not have enough space for all needed encodings.
Different schemes in use present potential conflict – different codes for the same symbol, different symbols with the same code if you deal with more than one scheme.
Enter unicode. See unicode.org

19. From unicode.org
Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language. The Unicode Standard has been adopted by such industry leaders as Apple, HP, IBM, JustSystems, Microsoft, Oracle, SAP, Sun, Sybase, Unisys and many others. Unicode is required by modern standards such as XML, Java, ECMAScript (JavaScript), LDAP, CORBA 3.0, WML, etc., and is the official way to implement ISO/IEC It is supported in many operating systems, all modern browsers, and many other products. The emergence of the Unicode Standard, and the availability of tools supporting it, are among the most significant recent global software technology trends.

20. Unicode There are three encoding forms: 8, 16, 32 bits
UTF-8 includes the ASCII codes
UTF-16 all commonly used symbols, other symbols available in pairs of 16-bit units
UTF-32 when size is not an issue. All symbols in 32 bit string of bits

21. Using unicode

22. Regular Expressions
Processing text often involves selecting for specific characteristics
Regular expressions
powerful tool for describing the characteristics of interest
Access in python: import re
Raw string notation: precede a string with r
r’\n’means backslash then n, not new line

23. Regular Expression special characters
‘^’ (Caret) Matches the start of the string
‘$” matches the end of the string, or just before newline at the end of a string
‘*’ match 0 or more repetitions of the preceding re. 0*1 matches any number of 0s followed by 1: 1, 01, 001, 0001, etc.
‘+’ matches 1 or more repetition matches 01, 001, 0001, etc., but not 1
‘?’ matches 0 or 1 repetitions. 0?1 matches 1 and 01 only
{m,n} matches between m and n repetitions. If no n specified, matches only exactly m repetitions. 0{2,4}1 matches 001, 0001, 00001
{m,n}? match as few as possible of these. 0{2,4}1 will match 001 if it is available, or 0001 if no 001 is available, or if no shorter string is available.
\ escape special character, so you can search for * or ? etc
[ ] used to indicate a set of characters. [abc] will match a or b or c
range: [0-9A-Za-z] will match any digit or letter, upper or lower case
Special characters lose meaning in set: [\*] matches \ or *
^ = negate the set [^0-9] will match anything except a digit
| means “or” A|B means the character A or the character B. Options are tested left to right and the search quits when a match is found. This gives priority to the symbol listed first.

24. Python re import nltk import re
wordlist = [w for w in nltk.corpus.words.words('en') if w.islower()]
print [w for w in wordlist if re.search('ed$', w)]
matches all words in the list that end in ed
Wildcard . matches any single character
Crossword match example:
[w for w in wordlist if re.search('^..j..t..$', w)]
Crossword match example:
['abjectly', 'adjuster', 'dejected', 'dejectly', 'injector', 'majestic', 'objectee', 'objector', 'rejecter', 'rejector', 'unjilted', 'unjolted', 'unjustly’]

25. Spot check
Your Turn: The caret symbol ^ matches the start of a string, just like the $ matches the end. What results do we get with the above example if we leave out both of these, and search for «..j..t..»?
Think about it first. What do you expect?
Then run it.
Crossword match example:
['abjectedness', 'abjection', 'abjective', 'abjectly', 'abjectness', 'adjection', 'adjectional', 'adjectival', 'adjectivally', 'adjective', 'adjectively', 'adjectivism', 'adjectivitis', 'adjustable', 'adjustably', 'adjustage', 'adjustation', 'adjuster', 'adjustive', 'adjustment', 'antejentacular', 'antiprojectivity', 'bijouterie', 'coadjustment', 'cojusticiar', 'conjective', 'conjecturable', 'conjecturably', 'conjectural', 'conjecturalist', 'conjecturality', 'conjecturally', 'conjecture', 'conjecturer', 'coprojector', 'counterobjection', 'dejected', 'dejectedly', 'dejectedness', 'dejectile', 'dejection', …

26. ? as optional character ? indicates 0 or 1 occurrences ^e-?mail$
matches either or
^[Ee]-?mail$
allows either upper or lower case E
Note that [^Ee] matches anything that is not E,e
the negation is inside the [ ]

27. Texting example
[w for w in wordlist if re.search('^[ghi][mno][jlk][def]$', w)
First letter from ghi, second from mno, then jlk, then def
Take away the ^ and $
['gold', 'golf', 'hold', 'hole']
'tinkerlike', 'tinkerly', 'tinkershire', 'tinkershue', 'tinkerwise', 'tinlet', 'titleholder', 'toolholder', 'toolholding', 'touchhole', 'trainless', 'traphole', 'trinkerman', 'trinket', 'trinketer', 'trinketry', 'trinkety', 'triole', 'trioleate', 'triolefin', 'trioleic’, …

28. Python use of re re.search(pattern, string[,flags])
scan through string looking for pattern. Return None if not found.
re.match(pattern, string)
if zero or more characters at the beginning of string match the re pattern, return a correstponding MatchObject instance. Return None if string does not match the pattern.
re.split(pattern,string)
Split string by occurrences of pattern.
from: some options not included

29. \w = word class: equivalent to
[a-zA-Z0-9_]
\W = complement of \w – all characters other than letters and digits
>>> re.split('\W+', 'Words, words, words.')
['Words', 'words', 'words', '']
>>> re.split('(\W+)', 'Words, words, words.')
['Words', ', ', 'words', ', ', 'words', '.', '']
>>> re.split('\W+', 'Words, words, words.', 1)
['Words', 'words, words.']
>>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE)
['0', '3', '9']

30. re.findall(pattern, string[,flags])
return all non-overlapping matches of pattern in string, as a list of strings. String scanned left-to-right. Matches returned in order found.

31. Applications of re Extract word pieces another
> word = 'supercalifragilisticexpialidocious'
>>> re.findall(r'[aeiou]', word)
['u', 'e', 'a', 'i', 'a', 'i', 'i', 'i', 'e', 'i', 'a', 'i', 'o', 'i', 'o', 'u']
>>> len(re.findall(r'[aeiou]', word)) 16
>>> wsj = sorted(set(nltk.corpus.treebank.words()))
>>> fd = nltk.FreqDist(vs for word in wsj
for vs in re.findall(r'[aeiou]{2,}', word))
>>> fd.items()
vu50390:ch3 lcassel$ python re2.py
[('io', 549), ('ea', 476), ('ie', 331), ('ou', 329), ('ai', 261), ('ia', 253), ('ee', 217), ('oo', 174), ('ua', 109), ('au', 106), ('ue', 105), ('ui', 95), ('ei', 86), ('oi', 65), ('oa', 59), ('eo', 39), ('iou', 27), ('eu', 18), ('oe', 15), ('iu', 14), ('ae', 11), ('eau', 10), ('uo', 8), ('ao', 6), ('oui', 6), ('eou', 5), ('uou', 5), ('uee', 4), ('aa', 3), ('ieu', 3), ('uie', 3), ('eei', 2), ('aia', 1), ('aii', 1), ('aiia', 1), ('eea', 1), ('iai', 1), ('iao', 1), ('ioa', 1), ('oei', 1), ('ooi', 1), ('ueui', 1), ('uu', 1)]

32. Spot check
Your Turn: In the W3C Date Time Format, dates are represented like this: Replace the ? in the following Python code with a regular expression, in order to convert the string ' ' to a list of integers [2009, 12, 31]:
[int(n) for n in re.findall(?, ' ')]

33. Processing some text
Noting redundancy in English and eliminating internal word vowels:
>>> regexp = r'^[AEIOUaeiou]+|[AEIOUaeiou]+$|[^AEIOUaeiou]'
>>> def compress(word):
pieces = re.findall(regexp, word)
return ''.join(pieces)
...
>>> english_udhr = nltk.corpus.udhr.words('English-Latin1')
>>> print nltk.tokenwrap(compress(w) for w in english_udhr[:75])
Unvrsl Dclrtn of Hmn Rghts Prmble Whrs rcgntn of the inhrnt dgnty and
of the eql and inlnble rghts of all mmbrs of the hmn fmly is the fndtn
of frdm , jstce and pce in the wrld , Whrs dsrgrd and cntmpt fr hmn
rghts hve rsltd in brbrs acts whch hve outrgd the cnscnce of mnknd ,
and the advnt of a wrld in whch hmn bngs shll enjy frdm of spch and

34. Tabulating combinations
>>> rotokas_words = nltk.corpus.toolbox.words('rotokas.dic')
>>> cvs = [cv for w in rotokas_words for cv in re.findall\(r'[ptksvr][aeiou]', w)]
>>> cfd = nltk.ConditionalFreqDist(cvs)
>>> cfd.tabulate()
a e i o u k p r s t v

35. Inspecting the words behind the numbers
>>> cv_word_pairs = [(cv, w) for w in rotokas_words
for cv in re.findall(r'[ptksvr][aeiou]', w)]
>>> cv_index = nltk.Index(cv_word_pairs)
>>> cv_index['su']
['kasuari']
>>> cv_index['po']
['kaapo', 'kaapopato', 'kaipori', 'kaiporipie', 'kaiporivira', 'kapo', 'kapoa', 'kapokao', 'kapokapo', 'kapokapo', 'kapokapoa', 'kapokapoa', 'kapokapora', 'kapokapora', 'kapokaporo', 'kapokaporo', 'kapokari', 'kapokarito', 'kapokoa', 'kapoo', 'kapooto', 'kapoovira', 'kapopaa', 'kaporo', 'kaporo', 'kaporopa', 'kaporoto', 'kapoto', 'karokaropo', 'karopo', 'kepo', 'kepoi', 'keposi', 'kepoto']

36. Stemming Simple approach: >>> def stem(word):
for suffix in ['ing', 'ly', 'ed', 'ious', 'ies', 'ive', 'es', 's', 'ment']:
if word.endswith(suffix):
return word[:-len(suffix)]
return word

37. Building a stemmer Build a disjunction of all suffixes
Take a look. What do we have here?
r – raw string. Interpret everything just as what you see.
^ from the beginning
. match anything
* repeat the match anything 0 or more times
(ing|ly|ed|ious|ies|ive|es|s|ment) – look for one of these
$ at the end of the string
‘processing’ -- the string
result =
re.findall(r'^.*(ing|ly|ed|ious|ies|ive|es|s|ment)$', 'processing')
['ing']

38. To get the whole word Need to add ?:
>>> re.findall(r'^.*(?:ing|ly|ed|ious|ies|ive|es|s|ment)$', 'processing')
['processing']

39. Split the word into stem and suffix
Some subtleties involved
>>> re.findall(r'^(.*)(ing|ly|ed|ious|ies|ive|es|s|ment)$', 'processing')
[('process', 'ing')]
Looks ok, but
>>> re.findall(r'^(.*)(ing|ly|ed|ious|ies|ive|es|s|ment)$', 'processes')
[('processe', 's')]
The * is a greedy operator. It takes as much as it can get.
>>> re.findall(r'^(.*?)(ing|ly|ed|ious|ies|ive|es|s|ment)$', 'processes')
[('process', 'es')]
*? is non greedy version.
>>> re.findall(r'^(.*?)(ing|ly|ed|ious|ies|ive|es|s|ment)?$', 'language')
[('language', '')]
? makes the suffix list optional, matches when none present

40. A stemming function >>> def stem(word):
regexp = r'^(.*?)(ing|ly|ed|ious|ies|ive|es|s|ment)?$'
stem, suffix = re.findall(regexp, word)[0]
return stem
...
>>> raw = """DENNIS: Listen, strange women lying in ponds distributing swords
... is no basis for a system of government. Supreme executive power derives from
... a mandate from the masses, not from some farcical aquatic ceremony."""
>>> tokens = nltk.word_tokenize(raw)
>>> [stem(t) for t in tokens]
['DENNIS', ':', 'Listen', ',', 'strange', 'women', 'ly', 'in', 'pond',
'distribut', 'sword', 'i', 'no', 'basi', 'for', 'a', 'system', 'of', 'govern',
'.', 'Supreme', 'execut', 'power', 'deriv', 'from', 'a', 'mandate', 'from',
'the', 'mass', ',', 'not', 'from', 'some', 'farcical', 'aquatic', 'ceremony', '.']
Note some strange “words” returned as the stem: basi from basis and deriv and execut etc.

41. The Porter Stemmer
Official home:
The python version

42. ( ) means only that part is returned
>>> from nltk.corpus import gutenberg, nps_chat
>>> moby = nltk.Text(gutenberg.words('melville-moby_dick.txt'))
>>> moby.findall(r"<a> (<.*>) <man>")
monied; nervous; dangerous; white; white; white; pious; queer; good;
mature; white; Cape; great; wise; wise; butterless; white; fiendish;
pale; furious; better; certain; complete; dismasted; younger; brave;
brave; brave; brave
>>> chat = nltk.Text(nps_chat.words())
>>> chat.findall(r"<.*> <.*> <bro>")
you rule bro; telling you bro; u twizted bro
>>> chat.findall(r"<l.*>{3,}")
lol lol lol; lmao lol lol; lol lol lol; la la la la la; la la la; la
la la; lovely lol lol love; lol lol lol.; la la la; la la la
( ) means only that part is returned

43. re.show Co{l}or{l}ess green ideas s{l}eep furious{l}y
import nltk, re
sent = "Colorless green ideas sleep furiously"
nltk.re_show('l',sent)
nltk.re_show('gree',sent)
Co{l}or{l}ess green ideas s{l}eep furious{l}y
Colorless {gree}n ideas sleep furiously

44. Word patterns >>> from nltk.corpus import brown
>>> hobbies_learned = nltk.Text(brown.words(categories=['hobbies', 'learned']))
>>> hobbies_learned.findall(r"<\w*> <and> <other> <\w*s>")
speed and other activities; water and other liquids; tomb and other
landmarks; Statues and other monuments; pearls and other jewels;
charts and other items; roads and other features; figures and other
objects; military and other areas; demands and other factors;
abstracts and other compilations; iron and other metals

45. Spot Check How would you find all instances of the pattern as x as y
example: as easy as pie
Can you handle this: as pretty as a picture

46. More on Stemming >>> porter = nltk.PorterStemmer()
>>> lancaster = nltk.LancasterStemmer()
>>> [porter.stem(t) for t in tokens]
['DENNI', ':', 'Listen', ',', 'strang', 'women', 'lie', 'in', 'pond',
'distribut', 'sword', 'is', 'no', 'basi', 'for', 'a', 'system', 'of', 'govern',
'.', 'Suprem', 'execut', 'power', 'deriv', 'from', 'a', 'mandat', 'from',
'the', 'mass', ',', 'not', 'from', 'some', 'farcic', 'aquat', 'ceremoni', '.']
>>> [lancaster.stem(t) for t in tokens]
['den', ':', 'list', ',', 'strange', 'wom', 'lying', 'in', 'pond', 'distribut',
'sword', 'is', 'no', 'bas', 'for', 'a', 'system', 'of', 'govern', '.', 'suprem',
'execut', 'pow', 'der', 'from', 'a', 'mand', 'from', 'the', 'mass', ',', 'not',
'from', 'som', 'farc', 'aqu', 'ceremony', '.']
>>> wnl = nltk.WordNetLemmatizer()
>>> [wnl.lemmatize(t) for t in tokens]
['DENNIS', ':', 'Listen', ',', 'strange', 'woman', 'lying', 'in', 'pond',
'distributing', 'sword', 'is', 'no', 'basis', 'for', 'a', 'system', 'of',
'government', '.', 'Supreme', 'executive', 'power', 'derives', 'from', 'a',
'mandate', 'from', 'the', 'mass', ',', 'not', 'from', 'some', 'farcical',
'aquatic', 'ceremony', '.']
Only keeps stems if in dictionary

47. Tokenizing We have done split, but it was not very complete.
Built in re abbreviation for any kind of white space: \s
>>> re.split(r'\s+', raw)
['Dennis:', 'Listen,', 'strange', 'women', 'lying', 'in', 'ponds', 'distributing', 'swords', 'is', 'no', 'basis', 'for', 'a', 'system', 'of', 'government.', 'Supreme', 'executive', 'power', 'derives', 'from', 'a', 'mandate', 'from', 'the', 'masses,', 'not', 'from', 'some', 'farcical', 'aquatic', 'ceremony.']
>>>

48. Tokenizing Split on anything other than a word character (A-Za-z0-9)
>>> re.split(r'\W+', raw)
['', 'When', 'I', 'M', 'a', 'Duchess', 'she', 'said', 'to', 'herself', 'not', 'in',
'a', 'very', 'hopeful', 'tone', 'though', 'I', 'won', 't', 'have', 'any', 'pepper',
'in', 'my', 'kitchen', 'AT', 'ALL', 'Soup', 'does', 'very', 'well', 'without',
'Maybe', 'it', 's', 'always', 'pepper', 'that', 'makes', 'people', 'hot', 'tempered',
'']
Note: I’M became I M
re.findall(r'\w+', raw)
Splits on the words, instead of the separators
«\w+|\S\w*»
will first try to match any sequence of word characters. If no match is found, it will try to match any non-whitespace character (\S is the complement of \s) followed by further word characters. This means that punctuation is grouped with any following letters (e.g. 's) but that sequences of two or more punctuation characters are separated.

49. Getting there >>> re.findall(r'\w+|\S\w*', raw)
["'When", 'I', "'M", 'a', 'Duchess', ',', "'", 'she', 'said', 'to', 'herself', ',',
'(not', 'in', 'a', 'very', 'hopeful', 'tone', 'though', ')', ',', "'I", 'won', "'t",
'have', 'any', 'pepper', 'in', 'my', 'kitchen', 'AT', 'ALL', '.', 'Soup', 'does',
'very', 'well', 'without', '-', '-Maybe', 'it', "'s", 'always', 'pepper', 'that',
'makes', 'people', 'hot', '-tempered', ',', "'", '.', '.', '.']
Now get internal marks – ‘M and ‘t

50. Regular expression symbols
Summary
Symbol Function
\b Word boundary (zero width)
\d Any decimal digit (equivalent to [0-9])
\D Any non-digit character (equivalent to [^0-9])
\s Any whitespace character (equivalent to [ \t\n\r\f\v]
\S Any non-whitespace character (equivalent to [^ \t\n\r\f\v])
\w Any alphanumeric character (equivalent to [a-zA-Z0-9_])
\W Any non-alphanumeric character (equivalent to [^a-zA-Z0-9_])
\t The tab character
\n The newline character

51. Tokenizer in Python
>>> text = 'That U.S.A. poster-print costs $ '
>>> pattern = r'''(?x) # set flag to allow verbose regexps
([A-Z]\.) # abbreviations, e.g. U.S.A.
... | \w+(-\w+)* # words with optional internal hyphens
... | \$?\d+(\.\d+)?%? # currency and percentages, e.g. $12.40, 82%
... | \.\.\ # ellipsis
... | [][.,;"'?():-_`] # these are separate tokens
... '''
>>> nltk.regexp_tokenize(text, pattern)
['That', 'U.S.A.', 'poster-print', 'costs', '$12.40', '...']

52. Spot Check
☼ Describe the class of strings matched by the following regular expressions.
[a-zA-Z]+
[A-Z][a-z]*
p[aeiou]{,2}t
\d+(\.\d+)?
([^aeiou][aeiou][^aeiou])*
\w+|[^\w\s]+
Test your answers using nltk.re_show().

53. Exercise for next week
◑ Read in some text from a corpus, tokenize it, and print the list of all wh-word types that occur. (wh-words in English are used in questions, relative clauses and exclamations: who, which, what, and so on.) Print them in order. Are any words duplicated in this list, because of the presence of case distinctions or punctuation?