Regular Expression Beautiful
Project description
reb -- Regular Expression Beautiful
To make infomation extraction with patterns easier, reb tries to improve traditional re in some ways:
- Maintainability
- Reusability
- Readability
In order for that, several ideas are intruduced:
- A pythonic pattern style
- Examples make read and write easier
- Return parse tree rather than "Match Object"
A simple case for first impression
reb helps write regular expressions in a compasable and more readable style. Take url parsing and extracting as an example.
According to rfc3986, a simple regular expression that matches urls:
^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
can be rewritten with reb like:
from reb import P # "P" for "Pattern"
scheme = P.n(P.nic(':/?#'), 1) + ':' # "nic" for "Not In Chars"
# "n" for "repeat for N times"
hier = P.n01('//' + P.n(P.nic('/?#'))) + P.n(P.nic('?#')) # "n01" repeat for zero or 1 time
query = P.n01('?' + P.n(P.nic('#')))
fragment = P.n01('#' + P.n(P.ANYCHAR))
url = P.tag(P.n01(scheme), tag='scheme') \
+ P.tag(hier, tag='hierachy') \
+ P.tag(query, tag='query') \
+ P.tag(fragment, tag='fragment')
And here is how the match goes. It can be seen by command line tool reb
:
As it shows, tagged partes are filled with different colors, so that scheme, hierachy, query, fragment in the url are extracted.
Work with examples
Sometimes it is also hard to "parse" reb patterns with eyes and figure out what cases it matches. Naming helps writing explicit expressions, and another way to make it more readable is to bind pattern with examples.
from reb import P
scheme = P.example(
P.n(P.nic(':/?#'), 1) + ':',
'http:',
'https:'
)
hier = P.example(
P.n01('//' + P.n(P.nic('/?#'))) + P.n(P.nic('?#')),
'//google.com',
'localhost',
'127.0.0.1:8080',
)
query = P.example(
P.n01('?' + P.n(P.nic('#'))),
'',
'?a=1',
'?a=1&b=2',
)
fragment = P.example(
P.n01('#' + P.n(P.ANYCHAR)),
'',
'#head'
)
url = P.tag(P.n01(scheme), tag='scheme') \
+ P.tag(hier, tag='hierachy') \
+ P.tag(query, tag='query') \
+ P.tag(fragment, tag='fragment')
In the code above, each part of the pattern are bound with examples.
Examples do not change matches and extractions, but during the declaration (creation) of the pattern, a validation is done, ensuring the pattern appears in each of the example given. With well selected examples, code readers (somebody else or maybe the author 3 months later) will get something on seeing them: maybe cases that the expression was derived from, or the purpose of it. It is designed as a mechanism to explicitly "encode" our thinkings together with patterns on writing, and trace them back on reading. Pattern authors can use examples to convey more information to readers, and "make an agreement" easier with maintainers. So examples work like comments, but well, "explicit is better than implicit".
Regular expressions are widely used in some information extraction tasks on natural languages. As corpus goes complex, the re we write tend to become big and long, which is difficult to read and understand, not to mention to modify them. Even if patterns are divided into small parts, it is still a hard job to maintain or extend them, partly because no alarm happens when good cases turn bad. So on the other hand, examples work as a sort of test. With a careless pattern modification, one of the example may fails the creation of the pattern, which in turns alarms coders to pay attention in the first place.
Match more than one groups with same tag
Queries in url can be joined with "&", like passing multiple arguments. What if we want to extract each argument and its value? We may want to write traditional re like this:
\?(?P<query>[^#&]*)(&(?P<query>[^#&]*))*
Unfortunately, it raises an re compile error, complaining that the same group "query" is declared more than once.
sre_constants.error: redefinition of group name 'query' as group 3; was group 1 at position 25
In information extraction area, it is a common demand to tag two or more different text segment with a same label. In reb, it can be done by tag, like:
from reb import P
scheme = P.example(
P.n(P.nic(':/?#'), 1) + ':',
'http:',
'https:'
)
hier = P.example(
P.n01('//' + P.n(P.nic('/?#'))) + P.n(P.nic('?#')),
'//google.com',
'localhost',
'127.0.0.1:8080',
)
a_query = P.tag(P.n(P.nic('#&')), tag='query')
query = P.example(
P.n01('?' + P.n(a_query + '&') + a_query),
'',
'?a=1',
'?a=1&b=2',
)
fragment = P.example(
P.n01('#' + P.n(P.ANYCHAR)),
'',
'#head'
)
url = P.tag(P.n01(scheme), tag='scheme') \
+ P.tag(hier, tag='hierachy') \
+ query \
+ P.tag(fragment, tag='fragment')
The green texts show that, each "a_query" is extracted and tagged.
Programming interface
Pattern primitives & Pattern object
reb.P
is the only entry for users to build a pattern object.
convention: lower case for primitive pattern builders, upper case for primitive patterns.
primitive | short for | traditional re counterpart |
---|---|---|
pattern | literals | |
ic | In Chars | [] |
nic | Not In Chars | [^] |
tag | just like group | |
n | repeat for N times | * or {n, m} |
n01 | repeat for 0 or 1 time | + |
any | | | |
onceeach | ||
example | ||
ANYCHAR | . | |
STARTING | ^ | |
ENDING | $ | |
NEWLINE | \n |
Pattern.extract
is the main method for pattern object. It returns a list of PTNode
objects.
class Pattern:
def extract(self, text: str) -> List[PTNode]:
"""Extract info from text by the pattern, and return every match, forming a parse tree"""
NOTE: versions till now do not comply with posix regular expression standard, nor python re interface
Parse Tree Node
class PTNode
PTNode.text
original text that the parse tree was extracted fromPTNode.string
same asPTNode.text
PTNode.content
a text segment that matches a patternPTNode.start()
start index of the matchPTNode.end()
end index of the matchPTNode.children
children nodes of the parse treePTNode.fetch(tag)
iterate all nodes with tagtag
in the parse treePTNode.pp()
pretty printPTNode.show()
print parse tree in details
Project details
Release history Release notifications | RSS feed
Download files
Download the file for your platform. If you're not sure which to choose, learn more about installing packages.