A regex answers a yes or no question - does the string match the pattern.
I want to separate the no's into two categories:
Here's an example (regular expression 01+2):
012 is valid
12 is invalid; it is not a prefix of a valid string
01 is invalid; it is a prefix of a valid string: 012
Can re do this? If not, is there a library that can make this distinction?
I second the recommendation of regex. The module is simply fantastic.
Here's an example of fuzzy matching with regex:
import regex
# traditional matching - three digits
r = '(?:\d\d\d)'
print regex.findall(r, '1xx22yy333zz')
## ['333']
# fuzzy matching - three digits, allow at most 2 deletions
r = '(?:\d\d\d){d<3}'
print regex.findall(r, '1xx22yy333zz')
## ['1', '22', '333']
The {d<3} part basically says "if we add one or two chars to that, that would be a match" - the same as point 3. in your question.
See http://pypi.python.org/pypi/regex for more info (look for 'Approximate "fuzzy" matching').
I can’t tell what you really want. Are you trying to understand incremental matches?
Perhaps you need to learn to use anchors properly. Just as you should never use the Java misnamed and deceptive matches method instead of its find method, you should probably eschew Python’s match method in favor of search, and for precisely the same reason.
Another possibility is to rewrite your pattern with optional portions that you can then inspect the success of.
Or perhaps you should look into the fuzzy matching support in Matthew Barnett’s replacement regex library, which you really should be using instead of the crufty old re.
I can’t tell what you’re realy asking, because you haven’t given examples of desired input and output.
Perhaps you need nothing more complex than (?=.*(?:ab|bc)).*a?b?c?, or spaced out:
(?x)
(?= .* (?: ab | bc) )
.*
a? b? c?
If you put a, b, and c into recursible subgroups, you wouldn’t even have to repeat yourself.
{d<3} means allow up to 2 deletions. Therefore the answer was an example of fuzzy matching, limited to 0, 1 or 2 deletions. Did you mean that since it forbade the other operations it was not an instance of pure fuzzy matching?{d<3} part corresponds to fuzzy matching; I hadn’t seen it previously. However, you really need to disabuse yourself of all this ʀᴇɢᴜʟᴀʀ ʟᴀɴɢᴜᴀɢᴇ jazz, because that has nothing to do with almost any current regex syntax or system, including both Java’s and Python’s. You need to learn how to get the existing system to do whatever it is you really want to do. Modern pattern matching systems are infinitely more powerful than all this ʀᴇɢᴜʟᴀʀ ʟᴀɴɢᴜᴀɢᴇ business; just learn how to use them.Correct me if I am wrong, I believe the original question was: for some regex and input string if the input string is a possible match (ie. adding more characters to the end of the string could make it a match).
Thus fuzzy matching isn't an answer because:
rgx = regex.compile('(abcdef){d<5}')
str = 'bcd'
will be a match as well
Boost C++ library (Also for Java/Javascript?) provides such an option: http://www.boost.org/doc/libs/1_31_0/libs/regex/doc/partial_matches.html
So does anyone know what is the best way to achieve this in Python?
foomatches the stringThe food is in the barn., for example, and it does so perfectly well. On the other hand, the patternthesefails to match that same string. Are you wanting something that stops partway through the pattern and tells you where it failed?match. End of story. These things become trivial when you use the proper interface, and ridiculous if you don’t. And your idea of precision does not seem to correspond with my own.s/is in the language/matches/g; s/is not in the language/doesn’t match/g