Backreferences in python ?

**Gerhard Häring** · Jan 23 '06, 03:45 PM

Re: Backreferences in python ?

Pankaj wrote:[color=blue]
> [...]
> *************** *************** **********
> What i tried in python was::
> *************** *************** **********
>
> f = open( "./1.c", "r")
> fNew = open( "./1_new.c", "w")
> for l in f:
> print l
> lineno = lineno + 1
> strToFind = "for\((.*)\;(.* )"
> [...][/color]

Regular expressions are not handled automatically in Python the way you
apparently think they are.

In Python, you will need to use the "re" module:

re — Regular expression operations

http://docs.python.org/lib/module-re.html

Source code: Lib/re/ This module provides regular expression matching operations similar to those found in Perl. Both patterns and strings to be searched can be Unicode strings ( str) as well as 8-...

-- Gerhard

**Pankaj** · Jan 23 '06, 05:25 PM

Re: Backreferences in python ?

My tries have with re have not yielded results::

{
strToFind = 'for*;*'

## Converting int to string, i.e. line no. to string
lineNoClone = lineno

pattern = "printf(\"" + str( lineNoClone) + "\"),"

regObj = re.compile( strToFind)

m = regObj.search( l)

if ( m != None ) :
subStrPattern1_ hasInitializati on = "\1"
#m.group(1)

subStrPattern2_ hasRestTillEnd = "\2"
#m.group(2)

strToReplace = "for(" + pattern +
subStrPattern1_ hasInitializati on + ";" + subStrPattern2_ hasRestTillEnd
fNew.write( regObj.sub( strToFind, strToReplace ) )
else:
fNew.write( l)
}

Here problem is , i am not getting backreferences using \1 and \2

The string : for( i =0; i < 10; i++)

is getting replace by: for *;* {

I don't believe this, they have given that \1 and \2 store
backreferences, then where r they??

**Pankaj** · Jan 23 '06, 05:55 PM

Re: Backreferences in python ?

I got my answer

if ( m != None ) :
subStrPattern1_ hasInitializati on = m.group(1)

subStrPattern2_ hasRestTillEnd = m.group(2)

str = subStrPattern1_ hasInitializati on +
subStrPattern2_ hasRestTillEnd
strToReplace = "for(" + pattern + str

This gave me my solution

But to tell u, i have not got that, why i should concatenate and then
only place it . while i was trying the same thing by concatenation it
straight to replace string, it was not working

Any body has reasons ???

**Duncan Booth** · Jan 23 '06, 06:05 PM

Re: Backreferences in python ?

Pankaj wrote:
[color=blue]
> Here problem is , i am not getting backreferences using \1 and \2
>[/color]

You wrote:[color=blue]
> subStrPattern1_ hasInitializati on = "\1"[/color]

"\1" is the way to create a string containing a control-A character. What
you actually wanted was a string containing a backslash and a "1", so you
need either:

"\\1"

or

r"\1"

Try using the print statement to see what all those strings you are
creating actually contain.

**Paul McGuire** · Jan 23 '06, 07:35 PM

Re: Backreferences in python ?

"Pankaj" <pankajgode@gma il.com> wrote in message
news:1138029493 .503320.178550@ o13g2000cwo.goo glegroups.com.. .[color=blue]
>
> I have something like below in perl and i am searching for equivalent
> in python:
>
> ::: Perl :::
> ***********
> while( <FILEHANDLE> )
> {
>
> line = $_;
>
> pattern = "printf$ \"$lineNo \" $,";
>
> line =~ s/"for(.*)\((*.)\ ;(.*)/for$1\($pattern $2\;$3/g;
> }
>
> This is used to
>
> search for : for ( i = 0; i < 10; i++)
> Replace with: for( printf( "10" ), i =0; i < 10; i++)
> Where 10 is the line no.
>[/color]

Here is a solution using pyparsing instead of re's. You're already used to
re's from using Perl, so you may be more comfortable using that tool in
Python as well. But pyparsing has some builtin features for pattern
matching, calling out to callback routines during parsing, and a lineno
function to report the current line number, all wrapped up in a simple
transformString method call.

Download pyparsing at http://pyparsing.sourceforge.net.

-- Paul

from pyparsing import Keyword,SkipTo, lineno,cStyleCo mment

# define grammar for a for statement
for_ = Keyword("for")
forInitializer = SkipTo(';').set ResultsName("in itializer")
forStmt = for_ + "(" + forInitializer + ';'

# ignore silly comments
forStmt.ignore( cStyleComment)

# setup a parse action that will insert line numbers
# parse actions are all called with 3 args:
# - the original string being parsed
# - the current parse location where the match occurred
# - the matching tokens
# if a value is returned from this function, transformString will
# insert it in place of the original content
def insertPrintStat ement(st,loc,to ks):
lineNumber = lineno(loc,st)
if toks[0]:
return r'print("%d\n") , %s' % (lineNumber,tok s[0])
else:
return r'print("%d\n") ' % lineNumber
forInitializer. setParseAction( insertPrintStat ement)

# transform some code
# this is how you would read in a whole file as a single string
#testdata = file(inputfilen ame).read()
# to read the entire file into a list of strings, do:
#testdata = file(inputfilen ame).readlines( )
# for now, just fake some source code
testData = """
for(i = 0; i <= 100; ++i)
{
/* some stuff */
}

for (;;;)
{
/* do this forever */
}

/* this for has been commented out
for(a = -1; a < 0; a++)
*/

"""

# use the grammar and the associated parse action to
# transform the source code
print forStmt.transfo rmString(testDa ta)

--------------------------
Gives:
for(print("2\n" ), i = 0; i <= 100; ++i)
{
/* some stuff */
}

for(print("7\n" );;;)
{
/* do this forever */
}

/* this for has been commented out
for(a = -1; a < 0; a++)
*/

**Giovanni Bajo** · Jan 23 '06, 08:15 PM

Re: Backreferences in python ?

Pankaj wrote:
[color=blue][color=green][color=darkred]
>>>> Perl :::[/color][/color]
> ***********
> while( <FILEHANDLE> )
> {
>
> line = $_;
>
> pattern = "printf$ \"$lineNo \" $,";
>
> line =~ s/"for(.*)\((*.)\ ;(.*)/for$1\($pattern $2\;$3/g;
> }
>
> This is used to
>
> search for : for ( i = 0; i < 10; i++)
> Replace with: for( printf( "10" ), i =0; i < 10; i++)
> Where 10 is the line no.[/color]

import re
import fileinput

for L in fileinput.input (inplace=True):
pattern = 'printf("%d"),' % input.filelinen o()
L = re.sub(r"for(.* )\((*.)\;(.*)", r"for\1\(%s\2;\ 3" % pattern, L)
print L,

or something
--
Giovanni Bajo

**Sion Arrowsmith** · Jan 24 '06, 09:35 AM

Re: Backreferences in python ?

Pankaj <pankajgode@gma il.com> wrote:[color=blue]
>search for : for ( i = 0; i < 10; i++)
>Replace with: for( printf( "10" ), i =0; i < 10; i++)
>Where 10 is the line no.[/color]
[color=blue]
>f = open( "./1.c", "r")
>fNew = open( "./1_new.c", "w")
>for l in f:
> print l
> lineno = lineno + 1
> strToFind = "for\((.*)\;(.* )"[/color]
[etc.][color=blue]
>search for : for ( i = 0; i < 10; i++)
>Replace with: for( printf( "10" ), i =0; i < 10; i++)[/color]

Ah, the dangers of thinking of all string manipulation as requiring
regexps, thanks to their ubiquity in Perl. Observe:[color=blue]
>search for : for ( i = 0; i < 10; i++)
>Replace with: for( printf( "10" ), i = 0; i < 10; i++)[/color]

All you need is:
strToFind = "for ("
strToReplace = 'for (printf( "+str(lineno)+' " ),'
# Note the use of '' to avoid the need to escape the "s
fNew.write(l.re place(strToFind , strToReplace)

(OK, maybe you do need the regexp if you've got any "for (;" loops,
or inconsitencies as to whether it's "for(" or "for (". But if a
simple string replace will do the job, use it.)

--
\S -- siona@chiark.gr eenend.org.uk -- http://www.chaos.org.uk/~sion/
___ | "Frankly I have no feelings towards penguins one way or the other"
\X/ | -- Arthur C. Clarke
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