regular expression: perl ==> python

[les_ander@yahoo.com]

Hi,
i am so use to perl's regular expression that i find it hard
to memorize the functions in python; so i would appreciate if
people can tell me some equivalents.

1) In perl:
$line = "The food is under the bar in the barn.";
if ( $line =~ /foo(.*)bar/ ) { print "got <$1>\n"; }

in python, I don't know how I can do this?
How does one capture the $1? (I know it is \1 but it is still not clear
how I can simply print it.
thanks

[Steven Bethard]

Re: regular expression: perl ==&gt; python

les_ander@yahoo .com wrote:[color=blue]
>
> 1) In perl:
> $line = "The food is under the bar in the barn.";
> if ( $line =~ /foo(.*)bar/ ) { print "got <$1>\n"; }
>
> in python, I don't know how I can do this?[/color]

I don't know Perl very well, but I believe this is more or less the
equivalent:
[color=blue][color=green][color=darkred]
>>> import re
>>> line = "The food is under the bar in the barn."
>>> matcher = re.compile(r'fo o(.*)bar')
>>> match = matcher.search( line)
>>> print 'got <%s>' % match.group(1)[/color][/color][/color]
got <d is under the bar in the >

Of course, you can do this in fewer lines if you like:
[color=blue][color=green][color=darkred]
>>> print 'got <%s>' % re.search(r'foo (.*bar)', line).group(1)[/color][/color][/color]
got <d is under the bar in the bar>

Steve

[Fredrik Lundh]

Re: regular expression: perl ==&gt; python

<les_ander@yaho o.com> wrote:
[color=blue]
> i am so use to perl's regular expression that i find it hard
> to memorize the functions in python; so i would appreciate if
> people can tell me some equivalents.
>
> 1) In perl:
> $line = "The food is under the bar in the barn.";
> if ( $line =~ /foo(.*)bar/ ) { print "got <$1>\n"; }
>
> in python, I don't know how I can do this?
> How does one capture the $1? (I know it is \1 but it is still not clear
> how I can simply print it.[/color]

in Python, the RE machinery returns match objects, which has methods
that let you dig out more information about the match. "captured groups"
are available via the "group" method:

m = re.search(..., line)
if m:
print "got", m.group(1)

see the regex howto (or the RE chapter in the library reference) for more
information:

http://www.amk.ca/python/howto/regex/

</F>

[JZ]

Re: regular expression: perl ==&gt; python

Dnia 21 Dec 2004 21:12:09 -0800, les_ander@yahoo .com napisa³(a):
[color=blue]
> 1) In perl:
> $line = "The food is under the bar in the barn.";
> if ( $line =~ /foo(.*)bar/ ) { print "got <$1>\n"; }
>
> in python, I don't know how I can do this?
> How does one capture the $1? (I know it is \1 but it is still not clear
> how I can simply print it.
> thanks[/color]

import re
line = "The food is under the bar in the barn."
if re.search(r'foo (.*)bar',line):
print 'got %s\n' % _.group(1)

--
JZ ICQ:6712522

http://zabiello.om

[Fredrik Lundh]

Re: regular expression: perl ==&gt; python

"JZ" <wnebfynj@mnovr yyb.pbz> wrote:
[color=blue]
> import re
> line = "The food is under the bar in the barn."
> if re.search(r'foo (.*)bar',line):
> print 'got %s\n' % _.group(1)[/color]

Traceback (most recent call last):
File "jz.py", line 4, in ?
print 'got %s\n' % _.group(1)
NameError: name '_' is not defined

</F>

[Doug Holton]

Re: regular expression: perl ==&gt; python

Fredrik Lundh wrote:[color=blue]
> "JZ" <wnebfynj@mnovr yyb.pbz> wrote:
>
>[color=green]
>>import re
>>line = "The food is under the bar in the barn."
>>if re.search(r'foo (.*)bar',line):
>> print 'got %s\n' % _.group(1)[/color]
>
>
> Traceback (most recent call last):
> File "jz.py", line 4, in ?
> print 'got %s\n' % _.group(1)
> NameError: name '_' is not defined[/color]

He was using the python interactive prompt, which I suspect you already
knew.

[JZ]

Re: regular expression: perl ==&gt; python

Dnia Wed, 22 Dec 2004 10:27:39 +0100, Fredrik Lundh napisa³(a):
[color=blue][color=green]
>> import re
>> line = "The food is under the bar in the barn."
>> if re.search(r'foo (.*)bar',line):
>> print 'got %s\n' % _.group(1)[/color]
>
> Traceback (most recent call last):
> File "jz.py", line 4, in ?
> print 'got %s\n' % _.group(1)
> NameError: name '_' is not defined[/color]

I forgot to add: I am using Python 2.3.4/Win32 (from ActiveState.com ). The
code works in my interpreter.

--
JZ

[Fredrik Lundh]

Re: regular expression: perl ==&gt; python

"JZ" wrote:
[color=blue][color=green][color=darkred]
> >> import re
> >> line = "The food is under the bar in the barn."
> >> if re.search(r'foo (.*)bar',line):
> >> print 'got %s\n' % _.group(1)[/color]
> >
> > Traceback (most recent call last):
> > File "jz.py", line 4, in ?
> > print 'got %s\n' % _.group(1)
> > NameError: name '_' is not defined[/color]
>
> I forgot to add: I am using Python 2.3.4/Win32 (from ActiveState.com ). The
> code works in my interpreter.[/color]

only if you type it into the interactive prompt. see:

http://www.python.org/doc/2.4/tut/node5.html#SECTION005110000000000000000

"In interactive mode, the last printed expression is assigned to the variable _.
This means that when you are using Python as a desk calculator, it is some-
what easier to continue calculations /.../"

the "_" symbol has no special meaning when you run a Python program, so the
"if re.search" construct won't work.

</F>

[JZ]

Re: regular expression: perl ==&gt; python

Dnia Wed, 22 Dec 2004 16:55:55 +0100, Fredrik Lundh napisa³(a):
[color=blue]
> the "_" symbol has no special meaning when you run a Python program,[/color]

That's right. So the final code will be:

import re
line = "The food is under the bar in the barn."
found = re.search('foo( .*)bar',line)
if found: print 'got %s\n' % found.group(1)

--
JZ ICQ:6712522

http://zabiello.com

[Nick Craig-Wood]

Re: regular expression: perl ==&gt; python

> 1) In perl:[color=blue]
> $line = "The food is under the bar in the barn.";
> if ( $line =~ /foo(.*)bar/ ) { print "got <$1>\n"; }
>
> in python, I don't know how I can do this?
> How does one capture the $1? (I know it is \1 but it is still not clear
> how I can simply print it.
> thanks[/color]


Fredrik Lundh <fredrik@python ware.com> wrote:[color=blue]
> "JZ" <wnebfynj@mnovr yyb.pbz> wrote:
>[color=green]
> > import re
> > line = "The food is under the bar in the barn."
> > if re.search(r'foo (.*)bar',line):
> > print 'got %s\n' % _.group(1)[/color]
>
> Traceback (most recent call last):
> File "jz.py", line 4, in ?
> print 'got %s\n' % _.group(1)
> NameError: name '_' is not defined[/color]

I've found that a slight irritation in python compared to perl - the
fact that you need to create a match object (rather than relying on
the silver thread of $_ (etc) running through your program ;-)

import re
line = "The food is under the bar in the barn."
m = re.search(r'foo (.*)bar',line)
if m:
print 'got %s\n' % m.group(1)

This becomes particularly irritating when using if, elif etc, to
match a series of regexps, eg

line = "123123"
m = re.search(r'^(\ d+)$', line)
if m:
print "int",int(m.gro up(1))
else:
m = re.search(r'^(\ d*\.\d*)$', line)
if m:
print "float",float(m .group(1))
else:
print "unknown thing", line

The indentation keeps growing which looks rather untidy compared to
the perl

$line = "123123";
if ($line =~ /^(\d+)$/) {
print "int $1\n";
}
elsif ($line =~ /^(\d*\.\d*)$/) {
print "float $1\n";
}
else {
print "unknown thing $line\n";
}

Is there an easy way round this? AFAIK you can't assign a variable in
a compound statement, so you can't use elif at all here and hence the
problem?

I suppose you could use a monstrosity like this, which relies on the
fact that list.append() returns None...

line = "123123"
m = []
if m.append(re.sea rch(r'^(\d+)$', line)) or m[-1]:
print "int",int(m[-1].group(1))
elif m.append(re.sea rch(r'^(\d*\.\d *)$', line)) or m[-1]:
print "float",flo at(m[-1].group(1))
else:
print "unknown thing", line

--
Nick Craig-Wood <nick@craig-wood.com> -- http://www.craig-wood.com/nick

[Fredrik Lundh]

Re: regular expression: perl ==&gt; python

Nick Craig-Wood wrote:
[color=blue]
> I've found that a slight irritation in python compared to perl - the
> fact that you need to create a match object (rather than relying on
> the silver thread of $_ (etc) running through your program ;-)[/color]

the old "regex" engine associated the match with the pattern, but that
approach isn't thread safe...
[color=blue]
> line = "123123"
> m = re.search(r'^(\ d+)$', line)
> if m:
> print "int",int(m.gro up(1))
> else:
> m = re.search(r'^(\ d*\.\d*)$', line)
> if m:
> print "float",float(m .group(1))
> else:
> print "unknown thing", line[/color]

that's not a very efficient way to match multiple patterns, though. a
much better way is to combine the patterns into a single one, and use
the "lastindex" attribute to figure out which one that matched. see

http://effbot.org/zone/xml-scanner.htm

for more on this topic.

</F>

[John Machin]

Re: regular expression: perl ==&gt; python


Fredrik Lundh wrote:[color=blue]
> "JZ" wrote:
>[color=green][color=darkred]
> > >> import re
> > >> line = "The food is under the bar in the barn."
> > >> if re.search(r'foo (.*)bar',line):
> > >> print 'got %s\n' % _.group(1)
> > >
> > > Traceback (most recent call last):
> > > File "jz.py", line 4, in ?
> > > print 'got %s\n' % _.group(1)
> > > NameError: name '_' is not defined[/color]
> >
> > I forgot to add: I am using Python 2.3.4/Win32 (from[/color][/color]
ActiveState.com ). The[color=blue][color=green]
> > code works in my interpreter.[/color]
>
> only if you type it into the interactive prompt. see:[/color]

No, it doesn't work at all, anywhere. Did you actually try this?
[color=blue]
>
>[/color]
http://www.python.org/doc/2.4/tut/no...00000000000000[color=blue]
>
> "In interactive mode, the last printed expression is assigned to[/color]
the variable _.[color=blue]
> This means that when you are using Python as a desk calculator,[/color]
it is some-[color=blue]
> what easier to continue calculations /.../"
>[/color]

In the 3 lines that are executed before the exception, there are *no*
printed expressions.

Python 2.4 (#60, Nov 30 2004, 11:49:19) [MSC v.1310 32 bit (Intel)] on
win32
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> import re
>>> line = "The food is under the bar in the barn."
>>> if re.search(r'foo (.*)bar',line):[/color][/color][/color]
.... print 'got %s\n' % _.group(1)
....
Traceback (most recent call last):
File "<stdin>", line 2, in ?
NameError: name '_' is not defined[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

[Fredrik Lundh]

Re: regular expression: perl ==&gt; python

John Machin wrote:[color=blue]
>[color=green][color=darkred]
>> > I forgot to add: I am using Python 2.3.4/Win32 (from ActiveState.com ). The
>> > code works in my interpreter.[/color]
>>
>> only if you type it into the interactive prompt. see:[/color]
>
> No, it doesn't work at all, anywhere. Did you actually try this?[/color]

the OP claims that it works in his ActiveState install (PythonWin?). maybe he
played with re.search before typing in the commands he quoted; maybe Python-
Win contains some extra hacks?

as I've illustrated earlier, it definitely doesn't work in a script executed by a standard
Python...

</F>

[John Machin]

Re: regular expression: perl ==&gt; python


Fredrik Lundh wrote:[color=blue]
> John Machin wrote:[color=green]
> >[color=darkred]
> >> > I forgot to add: I am using Python 2.3.4/Win32 (from[/color][/color][/color]
ActiveState.com ). The[color=blue][color=green][color=darkred]
> >> > code works in my interpreter.
> >>
> >> only if you type it into the interactive prompt. see:[/color]
> >
> > No, it doesn't work at all, anywhere. Did you actually try this?[/color]
>
> the OP claims that it works in his ActiveState install (PythonWin?).[/color]
maybe he[color=blue]
> played with re.search before typing in the commands he quoted; maybe[/color]
Python-[color=blue]
> Win contains some extra hacks?
>
> as I've illustrated earlier, it definitely doesn't work in a script[/color]
executed by a standard[color=blue]
> Python...
>
> </F>[/color]

It is quite possible that the OP played with re.search before before
typing in the commands he quoted; however *you* claimed that it [his
quoted commands] worked "only if you type it into the interactive
prompt". It doesn't work, in the unqualified sense that I understood.

Anyway, enough of punch-ups about how many dunces can angle on the hat
of a pun -- I did appreciate your other posting about multiple patterns
and "lastindex" ; thanks.

[Stephen Thorne]

Re: regular expression: perl ==&gt; python

On 22 Dec 2004 17:30:04 GMT, Nick Craig-Wood <nick@craig-wood.com> wrote:[color=blue]
> Is there an easy way round this? AFAIK you can't assign a variable in
> a compound statement, so you can't use elif at all here and hence the
> problem?
>
> I suppose you could use a monstrosity like this, which relies on the
> fact that list.append() returns None...
>
> line = "123123"
> m = []
> if m.append(re.sea rch(r'^(\d+)$', line)) or m[-1]:
> print "int",int(m[-1].group(1))
> elif m.append(re.sea rch(r'^(\d*\.\d *)$', line)) or m[-1]:
> print "float",flo at(m[-1].group(1))
> else:
> print "unknown thing", line[/color]

I wrote a scanner for a recursive decent parser a while back. This is
the pattern i used for using mulitple regexps, instead of using an
if/elif/else chain.

import re
patterns = [
(re.compile('^( \d+)$'),int),
(re.compile('^( \d+\.\d*)$'),fl oat),
]

def convert(s):
for regexp, action in patterns:
m = regexp.match(s)
if not m:
continue
return action(m.group( 1))
raise ValueError, "Invalid input %r, was not a numeric string" % (s,)

if __name__ == '__main__':
tests = [ ("123123",12312 3), ("123.123",123. 123), ("123.",123. ) ]
for input, expected in tests:
assert convert(input) == expected

try:
convert('')
convert('abc')
except:
pass
else:
assert None,"Should Raise on invalid input"


Of course, I wrote the tests first. I used your regexp's but I was
confused as to why you were always using .group(1), but decided to
leave it. I would probably actually send the entire match object to
the action. Using something like:
(re.compile('^( \d+)$'),lambda m:int(m.group(1 )),
and
return action(m)

but lambdas are going out fashion. :(

Stephen Thorne