Re ObjectsΒΆ

Re objects are memoized for efficiency, so they compile their pattern just once, regardless of how many times they’re mentioned in a program.

Note that the in test turns the sense of the matching around (compared to the standard re module). It asks “is the given string in the set of items this pattern describes?” To be fancy, the Re pattern is an intensionally defined set (namely “all strings matching the pattern”). This order often makes excellent sense whey you have a clear intent for the test. For example, “is the given string within the set of all legitimate commands?”

Second, the in test had the side effect of setting the underscore name _ to the result. Python doesn’t support en passant assignment–apparently, no matter how hard you try, or how much introspection you use. This makes it harder to both test and collect results in the same motion, even though that’s often exactly appropriate. Collecting them in a class variable is a fallback strategy (see the En Passant section below for a slicker one).

If you prefer the more traditional re calls:

if Re(pattern).search(some_string):
    print Re._[1]

Re works even better with named pattern components, which are exposed as attributes of the returned object:

person = 'John Smith 48'
if person in Re(r'(?P<name>[\w\s]*)\s+(?P<age>\d+)'):
    print, "is", Re._.age, "years old"
    print "don't understand '{}'".format(person)

One trick being used here is that the returned object is not a pure _sre.SRE_Match that Python’s re module returns. Nor is it a subclass. (That class appears to be unsubclassable.) Thus, regular expression matches return a proxy object that exposes the match object’s numeric (positional) and named groups through indices and attributes. If a named group has the same name as a match object method or property, it takes precedence. Either change the name of the match group or access the underlying property thus:

It’s possible also to loop over the results:

for found in Re('pattern (\w+)').finditer('pattern is as pattern does'):
    print found[1]

Or collect them all in one fell swoop:

found = Re('pattern (\w+)').findall('pattern is as pattern does')

Pretty much all of the methods and properties one can access from the standard re module are available.