Very often we need to look for the occurence of words in a string or group of words. We rely on regular expressions for such operations. A very common requirement is to look for the occurence of certain words in a paragraph or string. We can group the occurence by boolean operators AND, OR and NOT, allowing to search for certain words using boolean logic.
This class is created to do exactly that. It wraps up a complex boolean word expression, creating an internal regular expression, and provides methods allowing you to perform matches and searches on it.
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import re
class PyBoolReException(Exception):
def __init__(self, value):
self.value = value
def __str__(self):
return str(self.value)
class PyBoolRe:
""" A class to perform boolean word matches in
a string or paragraph. This class allows you to
perform complex matches in a string or group of
words by creating simple boolean expressions,
grouped by parantheses to create complex match
expressions.
Author: Anand B Pillai, http://tinyurl.com/yq3y
Copyright: None
LICENSE: GPL
Version: 0.2
Usage:
1. Create regular expressions using the boolean
keywords '|' and '&', standing for 'OR' and
'AND' respectively.
2. Use parantheses to group the boolean expressions
to create complex match expressions.
3. Caveats:
1. Fails for expressions with redundant parens such
as ((A | B)) etc.
Example:
p = PyBoolRe('Guido & Python')
s = 'Guido created Python'
mobject = p.match(s)
# Work with 'mobject' like you normally work with
# regular expression match objects
"""
def __init__(self, boolstr):
# Require whitespace before words?
self.__needspace = True
# whitespace re
self._wspre = re.compile('^\s*$')
# create regexp string
self.__rexplist = []
oparct = boolstr.count('(')
clparct = boolstr.count(')')
if oparct != clparct:
raise PyBoolReException, 'Mismatched parantheses!'
self.__parse(boolstr)
# if NOT is one of the members, reverse
# the list
# print self.__rexplist
if '!' in self.__rexplist:
self.__rexplist.reverse()
s = self.__makerexp(self.__rexplist)
# print s
self.__rexp = re.compile(s)
def match(self, data):
""" Match the boolean expression, behaviour
is same as the 'match' method of re """
return self.__rexp.match(data)
def search(self, data):
""" Search the boolean expression, behaviour
is same as the 'search' method of re """
return self.__rexp.search(data)
def __parse(self, s):
""" Parse the boolean regular expression string
and create the regexp list """
# The string is a nested parantheses with
# any character in between the parens.
scopy = s[:]
oparmatch, clparmatch = False, False
# Look for a NOT expression
index = scopy.rfind('(')
l = []
if index != -1:
oparmatch = True
index2 = scopy.find(')', index)
if index2 != -1:
clparmatch = True
newstr = scopy[index+1:index2]
# if the string is only of whitespace chars, skip it
if not self._wspre.match(newstr):
self.__rexplist.append(newstr)
replacestr = '(' + newstr + ')'
scopy = scopy.replace(replacestr, '')
self.__parse(scopy)
if not clparmatch and not oparmatch:
if scopy: self.__rexplist.append(scopy)
def is_inbetween(self, l, elem):
""" Find out if an element is in between
in a list """
index = l.index(elem)
if index == -1:
return False
if index>2:
if index in range(1, len(l) -1):
return True
else:
return False
else:
return True
def __makenotexpr(self, s):
""" Make a NOT expression """
if s.find('!') == 0:
return ''.join(('(?!', s[1:], ')'))
else:
return s
def __makerexp(self, rexplist):
""" Make the regular expression string for
the boolean match from the nested list """
is_list = True
if type(rexplist) is str:
is_list = False
elem = rexplist
elif type(rexplist) is list:
elem = rexplist[0]
if type(elem) is list:
elem = elem[0]
eor = False
if not is_list or len(rexplist) == 1:
eor = True
word_str = '.*'
s=''
# Implementing NOT
if elem == '!':
return ''.join(('(?!', self.__makerexp(rexplist[1:]), ')'))
# Implementing OR
elif elem.find(' | ') != -1:
listofors = elem.split(' | ')
for o in listofors:
index = listofors.index(o)
in_bet = self.is_inbetween(listofors, o)
if o:
o = self.__makenotexpr(o)
if in_bet:
s = ''.join((s, '|', word_str, o, '.*'))
else:
s = ''.join((s, word_str, o, '.*'))
# Implementing AND
elif elem.find(' & ') != -1:
listofands = elem.split(' & ')
for a in listofands:
index = listofands.index(a)
in_bet = self.is_inbetween(listofands, a)
if a:
a = self.__makenotexpr(a)
s = ''.join((s, word_str, a, '.*'))
else:
if elem:
elem = self.__makenotexpr(elem)
s = ''.join((elem, '.*'))
if eor:
return s
else:
return ''.join((s, self.__makerexp(rexplist[1:])))
if __name__=="__main__":
p = PyBoolRe('(!Guido)')
s1 = 'Guido invented Python and Larry invented Perl'
s2 = 'Larry invented Perl, not Python'
if p.match(s1):
print 'Match found for first string'
else:
print 'No match found for first string'
if p.match(s2):
print 'Match found for second string'
else:
print 'No match found for second string'
|
The first version had errors in parsing of the expression, so I re-wrote the parse() method in this version.
Implemented a basic NOT operator handling algorithm - 19/12/03 The script no longer parses whitespace.
-Anand
One Error and One more caveat. Error:
I believe that in the following code:
the line:
should read:
The way it currently reads, any | will always "or" with Null, and will thus always be true. (i.e. Guido | Larry is really: Null | Guido | Larry).
Caveat:
There is also another caveat. In a boolean statement such as (Larry Error:
I believe that in the following code:
the line:
should read:
The way it currently reads, any | will always "or" with Null, and will thus always be true. (i.e. Guido | Larry is really: Null | Guido | Larry).
Caveat:
There is also another caveat. In a boolean statement such as (Larry