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Old 10-31-2014, 06:09 AM   #1
jags1984
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Perl : Regex inside a variable


Hi,

I am doing a search a replace using regex in PERL. I am reading the search and replace string from a file storing in a variable and trying to replace it.

$srcstr='fopen\((\w+)\,(\w+)\)'; #read from a file
$deststr='fopen\($2\,$1)'; #read from a file

$line =~ s/$srcstr/$deststr/;

This search and replace doesnt work , when we use capturing and backreference as a variable in Perl.
 
Old 10-31-2014, 06:48 AM   #2
grail
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You might wish to clarify a little more on "doesn't work"? Are you getting no output, wrong output, error messages, etc...

Also, please enclose data or code in [code][/code] tags
 
Old 10-31-2014, 07:02 AM   #3
jags1984
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Quote:
print "Before: ", $line;
$srcstr='fopen\((\w+)\,(\w+)\)';
$deststr='fopen\($2\,$1)';
$line =~ s/$srcstr/$deststr/;
print "Src: $srcstr Dest: $deststr \n";
print "After: ", $line, "\n";
Expected Output
Before: fopen(fname,filehandle);
Src: fopen\((\w+)\,(\w+)\) Dest: fopen\($2\,$1)
After: fopen(filehandle,fname);

Actual Output
Before: fopen(fname,filehandle);
Src: fopen\((\w+)\,(\w+)\) Dest: fopen\($2\,$1)
After: fopen\($2\,$1);



I wanted it to work as expected.

Last edited by jags1984; 10-31-2014 at 07:05 AM.
 
Old 10-31-2014, 07:47 AM   #4
grail
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You may have to wait for one of the perl experts, but I think your issue is one of timing.

At the point where you set deststr your positional items have no values and hence the string is set literally.
However your srcstr is interpreted once inside the search, so it is grabbing all that matched and then changing it for the literal string.

There may be some perl magic one of the gurus knows or a simple step you need to include, so hopefully they will jump on and advise

I would check some of your escapes in the deststr as well as there are either some missing or too many in the first place.
 
Old 10-31-2014, 11:45 PM   #5
kevnuke
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Quote:
Originally Posted by grail View Post
..but I think your issue is one of timing.
Does Perl have coroutine functionality?
 
Old 11-01-2014, 06:03 AM   #6
jpollard
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Perl has nearly everything.

The problem is one of substitution. What you are trying to do is a double interpretation of the "$deststr". First to replace the variable with the string... then again to replace the back references.

The "s" operator does only single substitution, like all handling of " strings. The first parameter is then interpreted (again), but the second is not - it is presumed to be a constant.


One way to get this is:
Code:
print "Before: ", $line,"\n";
$srcstr='fopen\((\w+)\,(\w+)\)';
$deststr='fopen($2,$1)';
$s="\$line =~ s/$srcstr/$deststr/;";
eval $s;
#$line =~ s/$srcstr/$deststr/;
print "Src: $srcstr Dest: $deststr \n";
print "After: ", $line, "\n";
This is not necessarily the best way as it requires the $line, $srcstr and $deststr to be global.

Another way (which is more likely to be what you want) uses the "e" modifier (twice). This also requires a slight change in the specification of the value of $deststr:

Code:
print "Before: ", $line,"\n";
$srcstr='fopen\((\w+)\,(\w+)\)';
$deststr='"fopen($2,$1)"';
$line =~ s/$srcstr/$deststr/ee;
print "Src: $srcstr Dest: $deststr \n";
print "After: ", $line, "\n";
Note the value of $deststr - it now has embedded the evaluation with a quoted string( a '"..."'). This is necessary because the e modifier causes the value to evaluated - causing string substitution which handles the $1 and $2 substitutions, then evaluates that (which removes the "..."), which is then used for the replacement string.

If the " quoting in $deststr is missing, then it will evaluate the string as the perl expression fopen(filehandle,fname), which would return nothing (usually, as the fname just might not exist). With the quotes the evaluation returns the string you want.

Last edited by jpollard; 11-01-2014 at 06:09 AM.
 
2 members found this post helpful.
Old 11-01-2014, 06:26 AM   #7
smallpond
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Edit: never mind - jpollard had better suggestion

Last edited by smallpond; 11-01-2014 at 06:29 AM.
 
Old 11-02-2014, 11:57 PM   #8
jags1984
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Thankyou jpollard,


It works
 
  


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