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Text Parsing in Python
- Gayatri Nittala
- Madhubala Vasireddy
Text Parsing
► The
three W’s!
► Efficiency and Perfection
What is Text Parsing?
► common
programming task
► extract or split a sequence of characters
Why is Text Parsing?
► Simple
file parsing
 A tab separated file
► Data
extraction
 Extract specific information from log file
► Find
and replace
► Parsers – syntactic analysis
► NLP
 Extract information from corpus
 POS Tagging
Text Parsing Methods
► String
Functions
► Regular Expressions
► Parsers
String Functions
► String
module in python
 Faster, easier to understand and maintain
► If
you can do, DO IT!
► Different built-in functions




Find-Replace
Split-Join
Startswith and Endswith
Is methods
Find and Replace
► find,
index, rindex, replace
► EX: Replace a string in all files in a directory
files = glob.glob(path)
for line in fileinput.input(files,inplace=1):
lineno = 0
lineno = string.find(line, stext)
if lineno >0:
line =line.replace(stext, rtext)
sys.stdout.write(line)
startswith and endswith
► Extract
quoted words from the given text
myString = "\"123\"";
if (myString.startswith("\""))
print "string with double quotes“
► Find
if the sentences are interrogative or
exclamative
► What
an amazing game that was!
► Do you like this?
endings = ('!', '?')
sentence.endswith(endings)
isMethods
► to
check alphabets, numerals, character
case etc
 m = 'xxxasdf ‘
 m.isalpha()
 False
Regular Expressions
► concise
way for complex patterns
► amazingly powerful
► wide variety of operations
► when you go beyond simple, think about
regular expressions!
Real world problems
► Match
IP Addresses, email addresses, URLs
► Match balanced sets of parenthesis
► Substitute words
► Tokenize
► Validate
► Count
► Delete duplicates
► Natural Language processing
RE in Python
► Unleash
the power - built-in re module
► Functions
 to compile patterns
► complie
 to perform matches
►
match, search, findall, finditer
 to perform opertaions on match object
►
group, start, end, span
 to substitute
►
►-
sub, subn
Metacharacters
Compiling patterns
► re.complile()
► pattern




for IP Address
^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$
^\d+\.\d+\.\d+\.\d+$
^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$
^([01]?\d\d?|2[0-4]\d|25[0-])\.
([01]?\d\d?|2[0-4]\d|25[0-5])\.
([01]?\d\d?|2[0-4]\d|25[0-5])\.
([01]?\d\d?|2[0-4]\d|25[0-5])$
Compiling patterns
► pattern
for matching parenthesis
 \(.*\)
 \([^)]*\)
 \([^()]*\)
Substitute
Perform several string substitutions on a given string
import re
def make_xlat(*args, **kwargs):
adict = dict(*args, **kwargs)
rx = re.compile('|'.join(map(re.escape, adict)))
def one_xlate(match):
return adict[match.group(0)]
def xlate(text):
return rx.sub(one_xlate, text)
return xlate
►
Count
► Split
and count words in the given text
 p = re.compile(r'\W+')
 len(p.split('This is a test for split().'))
Tokenize
► Parsing




and Natural Language Processing
s = 'tokenize these words'
words = re.compile(r'\b\w+\b|\$')
words.findall(s)
['tokenize', 'these', 'words']
Common Pitfalls
► operations
on fixed strings, single character
class, no case sensitive issues
► re.sub() and string.replace()
► re.sub() and string.translate()
► match vs. search
► greedy vs. non-greedy
PARSERS
► Flat
and Nested texts
► Nested tags, Programming language
constructs
► Better to do less than to do more!
Parsing Non flat texts
► Grammar
► States
► Generate
tokens and Act on them
► Lexer - Generates a stream of tokens
► Parser - Generate a parse tree out of the
tokens
► Lex and Yacc
Grammar Vs RE
►
Floating Point
#---- EBNF-style description of Python ---#
floatnumber ::= pointfloat | exponentfloat
pointfloat ::= [intpart] fraction | intpart "."
exponentfloat ::= (intpart | pointfloat) exponent
intpart
::= digit+
fraction
::= "." digit+
exponent
::= ("e" | "E") ["+" | "-"] digit+
digit
::= "0"..."9"
Grammar Vs RE
pat = r'''(?x)
(
(
(
# exponentfloat
# intpart or pointfloat
# pointfloat
(\d+)?[.]\d+ # optional intpart with fraction
|
\d+[.]
# intpart with period
)
# end pointfloat
|
\d+
# intpart
)
# end intpart or pointfloat
[eE][+-]?\d+
# exponent
)
# end exponentfloat
|
(
# pointfloat
(\d+)?[.]\d+
# optional intpart with fraction
|
\d+[.]
# intpart with period
)
# end pointfloat
'''
PLY - The Python Lex and Yacc
► higher-level
and cleaner grammar language
► LALR(1) parsing
► extensive input validation, error reporting,
and diagnostics
► Two moduoles lex.py and yacc.py
Using PLY - Lex and Yacc
► Lex:
► Import
the [lex] module
► Define a list or tuple variable 'tokens', the
lexer is allowed to produce
► Define tokens - by assigning to a specially
named variable ('t_tokenName')
► Build the lexer
 mylexer = lex.lex()
 mylexer.input(mytext) # handled by yacc
Lex
t_NAME
= r'[a-zA-Z_][a-zA-Z0-9_]*'
def t_NUMBER(t):
r'\d+'
try:
t.value = int(t.value)
except ValueError:
print "Integer value too large", t.value
t.value = 0
return t
t_ignore = " \t"
Yacc
► Import
the 'yacc' module
► Get a token map from a lexer
► Define a collection of grammar rules
► Build the parser
 yacc.yacc()
 yacc.parse('x=3')
Yacc
► Specially
named functions having a 'p_' prefix
def p_statement_assign(p):
'statement : NAME "=" expression'
names[p[1]] = p[3]
def p_statement_expr(p):
'statement : expression'
print p[1]
Summary
String Functions
A thumb rule - if you can do, do it.
► Regular Expressions
Complex patterns - something beyond simple!
► Lex and Yacc
Parse non flat texts - that follow some rules
►
References
► http://docs.python.org/
► http://code.activestate.com/recipes/langs/python/
► http://www.regular-expressions.info/
► http://www.dabeaz.com/ply/ply.html
► Mastering
Regular Expressions by Jeffrey E F.
Friedl
► Python Cookbook by Alex Martelli, Anna Martelli &
David Ascher
► Text processing in Python by David Mertz
Thank You
Q&A