Match groups in Python
Starting Python 3.8
, and the introduction of assignment expressions (PEP 572) (:=
operator), we can now capture the condition value re.search(pattern, statement)
in a variable (let's all it match
) in order to both check if it's not None
and then re-use it within the body of the condition:
if match := re.search('I love (\w+)', statement):
print(f'He loves {match.group(1)}')
elif match := re.search("Ich liebe (\w+)", statement):
print(f'Er liebt {match.group(1)}')
elif match := re.search("Je t'aime (\w+)", statement):
print(f'Il aime {match.group(1)}')
You could create a little class that returns the boolean result of calling match, and retains the matched groups for subsequent retrieval:
import re
class REMatcher(object):
def __init__(self, matchstring):
self.matchstring = matchstring
def match(self,regexp):
self.rematch = re.match(regexp, self.matchstring)
return bool(self.rematch)
def group(self,i):
return self.rematch.group(i)
for statement in ("I love Mary",
"Ich liebe Margot",
"Je t'aime Marie",
"Te amo Maria"):
m = REMatcher(statement)
if m.match(r"I love (\w+)"):
print "He loves",m.group(1)
elif m.match(r"Ich liebe (\w+)"):
print "Er liebt",m.group(1)
elif m.match(r"Je t'aime (\w+)"):
print "Il aime",m.group(1)
else:
print "???"
Update for Python 3 print as a function, and Python 3.8 assignment expressions - no need for a REMatcher class now:
import re
for statement in ("I love Mary",
"Ich liebe Margot",
"Je t'aime Marie",
"Te amo Maria"):
if m := re.match(r"I love (\w+)", statement):
print("He loves", m.group(1))
elif m := re.match(r"Ich liebe (\w+)", statement):
print("Er liebt", m.group(1))
elif m := re.match(r"Je t'aime (\w+)", statement):
print("Il aime", m.group(1))
else:
print()
Less efficient, but simpler-looking:
m0 = re.match("I love (\w+)", statement)
m1 = re.match("Ich liebe (\w+)", statement)
m2 = re.match("Je t'aime (\w+)", statement)
if m0:
print("He loves", m0.group(1))
elif m1:
print("Er liebt", m1.group(1))
elif m2:
print("Il aime", m2.group(1))
The problem with the Perl stuff is the implicit updating of some hidden variable. That's simply hard to achieve in Python because you need to have an assignment statement to actually update any variables.
The version with less repetition (and better efficiency) is this:
pats = [
("I love (\w+)", "He Loves {0}" ),
("Ich liebe (\w+)", "Er Liebe {0}" ),
("Je t'aime (\w+)", "Il aime {0}")
]
for p1, p3 in pats:
m = re.match(p1, statement)
if m:
print(p3.format(m.group(1)))
break
A minor variation that some Perl folk prefer:
pats = {
"I love (\w+)" : "He Loves {0}",
"Ich liebe (\w+)" : "Er Liebe {0}",
"Je t'aime (\w+)" : "Il aime {0}",
}
for p1 in pats:
m = re.match(p1, statement)
if m:
print(pats[p1].format(m.group(1)))
break
This is hardly worth mentioning except it does come up sometimes from Perl programmers.