[Tutor] Reg. Expressions Parenthesis
Hey guys, girls, hope everyone is doing well.
Here's my question, when using Regular Expressions, the docs say when
using parenthesis, it "captures" the data. This has got me confused
(doesn't take much), can someone explain this to me, please??
Here's an example to use. It's kinda long, so, if you'd rather provide
your own shorter ex, that'd be fine. Thanks for any help as always.
From: [\w\s]+?<([\w\-][\w\-\.]+@[\w\-][\w\-\.]+[a-zA-Z]{1,4})>
From: matches the literal text "From: "
[\w\s]+? matches one or more consecutive word characters or space
characters. The question mark makes the match non-greedy, so it will
match as few characters as possible while still allowing the whole
regular expression to match (in this case, it's probably not necessary,
but it does make the match more efficient since the thing that comes
immediately afterwards is not a word character or space character).
< matches a literal less-than sign (opening angle bracket)
The same regular expression you had before (without From: and
without parenthesis) is now surrounded by parentheses. This makes it a
capturing group, so you can call m.group(1) to get the text matched by
that part of the regex.
> matches a literal greater-than sign
Thanks,
Chris Kavanagh
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Re: [Tutor] Reg. Expressions Parenthesis
On Tue, Jan 17, 2012 at 3:07 AM, Chris Kavanagh wrote:
> Hey guys, girls, hope everyone is doing well.
>
> Here's my question, when using Regular Expressions, the docs say when
> using parenthesis, it "captures" the data. This has got me confused
> (doesn't take much), can someone explain this to me, please??
>
> Here's an example to use. It's kinda long, so, if you'd rather provide
> your own shorter ex, that'd be fine. Thanks for any help as always.
>
Here's a quick example:
import re
data = 'Wayne Werner fake-phone: 501-555-1234, fake-SSN: 123-12-1234'
parsed = re.search('([\d]{3})-([\d]{3}-[\d]{4})', data)
print(parsed.group())
print(parsed.groups())
parsed = re.search('[\d]{3}-[\d]{3}-[\d]{4}', data)
print(parsed.group())
print(parsed.groups())
You'll notice that you can access the individual clusters using the
.groups() method. This makes capturing the individual groups pretty easy.
Of course, capturing isn't just for storing the results. You can also use
the captured group later on.
Let's say, for some fictitious reason you want to find every letter that
appears as a double in some data. If you were to do this the "brute force"
way you'd pretty much have to do something like this:
for i in range(len(data)-1):
found = []
if data[i] == data[i+1]:
if not data[i] in found:
found.append(i)
print(found)
The regex OTOH looks like this:
In [29]: data = 'aaabababbcacacceadbacdb'
In [32]: parsed = re.findall(r'([a-z])\1', data)
In [33]: parsed
Out[33]: ['a', 'b', 'c']
Now, that example was super contrived and also simple. Very few real-world
applications will be as simple as that one - usually you have much crazier
specifications, like find every person who has blue eyes AND blue hair, but
only if they're left handed. Assuming you had data that looked like this:
NameEye ColorHair Color Handedness Favorite type of potato
WayneBlue BrownDexter Mashed
Sarah Blue Blonde SinisterSpam(?)
Kane Green White Dexter None
Kermit Blue Blue SinisterIdaho
You could parse out the data using captures and backrefrences [1].
HTH,
Wayne
[1] In this situation, of course, regex is overkill. It's easier to just
.split() and compare. But if you're parsing something really nasty like EDI
then sometimes a regex is just the best way to go[2].
[2] When people start to understand regexes they're like the proverbial man
who only has a hammer. As Jamie Zawinski said[3], "Some people, when
confronted with a problem, think
“I know, I'll use regular expressions.” Now they have two problems." I've
come across very few occasions that regexes were actually useful, and it's
usually extracting very specifically formatted data (money, phone numbers,
etc.) from copious amounts of text. I've not yet had a need to actually
process words with it. Especially using Python.
[3]http://regex.info/blog/2006-09-15/247
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[Tutor] Virtual pet cat program
I am trying to code a virtual pet cat program. Heres what I have come up with
so far:
#Pet cat program
#the user chooses a cat of their choice; choosing attributes such as colour,
name etc
#the user then has to look after the cat and keep it alive
#create a class for the cat
class Cat(object):
"""A virtual pet cat"""
def __init__(self, name, hunger = 5, boredom = 0, tiredness = 0):
print("Meww!")
self.name = name
self.hunger = hunger
self.boredom = boredom
self.tiredness = tiredness
def __str__(self):
att = "Feline friend\n"
att += "Name: " + self.name + "\n"
if self.hunger >= 2:
print("Your pet is hungry.")
elif self.boredom >= 2:
print("Your pet is bored.")
elif self.tiredness >= 2:
print("Your pet is tired.")
return att
def __pass_time(self):
self.hunger += 1
self.boredom += 1
self.tiredness += 1
def eat(self, food = 5):
print("Pu purrr!")
self.hunger -= food
if self.hunger < 0:
self.hunger = 0
self.__pass_time()
def play(self, fun = 5):
print("Chirrup!")
self.boredom -= fun
if self.boredom < 0:
self.boredom = 0
self.__pass_time()
def rest(self, sleep = 5):
print("...")
self.tiredness -= sleep
if self.tiredness < 0:
self.tiredness = 0
self.__pass_time()
#introduce and explain the program to the user
input("Welcome to the pet cat simulation program. You will get the opportunity
to look after a cat of choice. Press enter to begin.")
#get the users chosen attributes for their cat
name = input("Please enter a name for your cat: ")
user_cat = Cat(name)
#taking care of the cat loop
choice = None
while choice != "0":
print("What would you like to do? (Enter 1 to feed the cat, enter 2 to
play with the cat, enter 3 to leave the cat to rest or press 0 to exit the
program.")
choice = input(">")
if choice == "1":
user_cat.eat
print(user_cat)
elif choice == "2":
user_cat.play
print(user_cat)
elif choice == "3":
user_cat.rest
print(user_cat)
input("Press enter to exit...")
...Then I try to run the program:
Welcome to the pet cat simulation program. You will get the opportunity to look
after a cat of choice. Press enter to begin.
Please enter a name for your cat: X
Meww!
What would you like to do? (Enter 1 to feed the cat, enter 2 to play with the
cat, enter 3 to leave the cat to rest or press 0 to exit the program.
>1
Your pet is hungry.
Feline friend
Name: X
What would you like to do? (Enter 1 to feed the cat, enter 2 to play with the
cat, enter 3 to leave the cat to rest or press 0 to exit the program.
>1
Your pet is hungry.
Feline friend
Name: X
What would you like to do? (Enter 1 to feed the cat, enter 2 to play with the
cat, enter 3 to leave the cat to rest or press 0 to exit the program.
>1
Your pet is hungry.
Feline friend
Name: X
What would you like to do? (Enter 1 to feed the cat, enter 2 to play with the
cat, enter 3 to leave the cat to rest or press 0 to exit the program.
>1
Your pet is hungry.
Feline friend
Name: X
What would you like to do? (Enter 1 to feed the cat, enter 2 to play with the
cat, enter 3 to leave the cat to rest or press 0 to exit the program.
>0
Press enter to exit...
As you can see, no matter how many times I 'feed' the cat, its always hungry.
Can someone please tell me why it wont update?
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Re: [Tutor] Virtual pet cat program
>As you can see, no matter how many times I 'feed' the cat, its always hungry.
>Can someone please tell me why it wont update?
>while choice != "0":
>print("What would you like to do? (Enter 1 to feed the cat, enter 2 to
> play with the cat, enter 3 to leave the cat to rest or press 0 to exit the
> program.")
>choice = input(">")
>if choice == "1":
>user_cat.eat
>print(user_cat)
>elif choice == "2":
>user_cat.play
>print(user_cat)
>elif choice == "3":
>user_cat.rest
>print(user_cat)
You are not calling the function, just referring to it.
>>> user_cat.eat
>
You should be calling user_cat.eat() or user_cat.eat( 10 ) to actually perform
the action you desire.
Ramit
Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology
712 Main Street | Houston, TX 77002
work phone: 713 - 216 - 5423
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Re: [Tutor] Class vs. instance
Hello!
Here is another one.
class A:
def __init__(self, data):
self.data = data
print self.data
I'm trying to understand this function-like syntax:
A('foo').__init__(42)
A(12).data
What are we actually calling this way?
Are there any other ways to get the same result?
Cheers.
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Re: [Tutor] Class vs. instance
On 01/17/2012 09:13 PM, Stayvoid wrote:
Hello!
Here is another one.
class A:
def __init__(self, data):
self.data = data
print self.data
I'm trying to understand this function-like syntax:
A('foo').__init__(42)
A(12).data
What are we actually calling this way?
Are there any other ways to get the same result?
The first line creates an instance of class A, then immediately calls
the method __init__() as though it were a normal function. It then
discards the object.
You should only call __init__() from within another class' __init__().
It's called automatically when an object is created; leave it at that.
You also usually want to keep the instance around, and use it more than
once. Otherwise you could just use ordinary functions and dispense with
the confusion.
A(12) creates an object, then you reference the data attribute of that
object, then you throw them both out. Not much use there either.
Try something like:
obj= A(49)
print obj.data
obj2 = A("artichoke")
obj.data = 99
print obj.data
print obj2.data
Two more things. Remove the print statement from methods like
__init__(), unless it's just for debugging purposes. And add a base
class of object to your class definition, so that a new-style class is
created. When you get to more advanced usage, it'll make a difference,
and you might as well use the version of class that'll still work in
Python 3.x.
class A(object):
.
--
DaveA
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[Tutor] Reg Ex Parentheses
Hey guys, girls, hope everyone is doing well.
Here's my question, when using Regular Expressions, the docs say when
using parenthesis, it "captures" the data. This has got me confused
(doesn't take much), can someone explain this to me, please??
Here's an example to use. It's kinda long, so, if you'd rather provide
your own shorter ex, that'd be fine. Thanks for any help as always.
From: [\w\s]+?<([\w\-][\w\-\.]+@[\w\-][\w\-\.]+[a-zA-Z]{1,4})>
From: matches the literal text "From: "
[\w\s]+? matches one or more consecutive word characters or space
characters. The question mark makes the match non-greedy, so it will
match as few characters as possible while still allowing the whole
regular expression to match (in this case, it's probably not necessary,
but it does make the match more efficient since the thing that comes
immediately afterwards is not a word character or space character).
< matches a literal less-than sign (opening angle bracket)
The same regular expression you had before (without From: and
without parenthesis) is now surrounded by parentheses. This makes it a
capturing group, so you can call m.group(1) to get the text matched by
that part of the regex.
> matches a literal greater-than sign
Thanks,
Chris Kavanagh
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