I am trying to select the below value from database:
Reporting that one of #its many problems had been the recent# extended
sales slump in women's apparel, the seven-store retailer said it would
start a three-month liquidation sale in all of its stores.~(A) its
many problems had been the recent~(B) its many problems has been the
recently~(C) its many problems is the recently~(D) their many problems
is the recent~(E) their many problems had been the recent~
i am selecting this value in variable $ques and then selecting a text as below:
$ques=~s/^(.*?)\#(.*?)\#(.*?)$/$2/;
Now, while replacing the ~ character in the string by
$3=~s/~/\n/g; ---->line 171
and running the script, I am getting one error as:
Modification of a read-only value attempted at main.pl line 171
I want to replace all the ~ character with '\n' and print the final value. Please suggest how to do it.
*I have researched this on net, but got confused that how to handle these read only variables.
You've already got a good explanation of the problem from José Castro. But there's another solution if you're using a recent-ish version of Perl (Update: having checked more carefully, I find that means 5.14+). The /r argument to the substitution operator will copy your string, make the substitution on the copy and then return that altered value.
So you could write:
my $new_value = $3 =~ s/~/\n/rg;
It sounds like what you really want in this case is split rather than regular expression capture groups:
my #parts = split(/#/, $ques);
$parts[2] =~ s/~/\n/g;
It makes the intent of your code clearer since you are, in fact, splitting on # symbols.
Just like you say, the special variables $1, $2, etc., are read-only, and that means that you can't perform that substitution on them.
Performing the substitution on $ques will do what you need:
$ques =~ s/~/\n/g;
print $ques;
Do note that in the earlier substitution that you're performing on $ques you're getting rid of all the ~ characters.
Related
When I run a file which is begin with #!/usr/bin/perl -w, I get a error:
syntax error at line 153, near "=~ ?"
I try to add "#!/bin/bash", this error is not append, but I get another
error:
"line 34: syntax error near unexpected token `('"
line 153 in my file:
($output_volume =~ ?^([\S]+).mnc?) && ($base_name = $1) ||
die "sharpen_volume failed: output volume does not appear to be"
." a minc volume.\n";
line34 in my file:
use MNI::Startup qw(nocputimes);
$output_volume =~ ?^([\S]+).mnc?
This used to be valid perl and thus might appear in old code and instructional material.
From perlop:
In the past, the leading m in m?PATTERN? was optional, but omitting it would produce a deprecation warning. As of v5.22.0, omitting it produces a syntax error. If you encounter this construct in older code, you can just add m.
That is Perl code so the first error message is meaningful.
With delimiters other than // in the match operator you must have the explicit m for it, so
$output_volume =~ m?^([\S]+).mnc?
It is only with // delimiters that the m may be omitted; from Regex Quote-Like Operators (perlop)
If "/" is the delimiter then the initial m is optional.
See perlretut for a tutorial introduction to regex and perlre for reference.
Also note that the particular delimiters of ? trigger specific regex behavior in a special case. This is discussed by the end of the documentation section in perlop linked above.
You already have two answers that explain the problem.
? ... ? is no longer valid syntax for a match operator. You need m? ... ? instead.
Until Perl 5.22, your syntax generated a warning. Now it's a fatal error (which is what you are seeing). So I assume you're now running this on a more recent version of Perl.
There are, however, a few other points it is probably worth making.
You say you tried to investigate this by changing the first line of your file from #!/usr/bin/perl -w to #!/bin/bash. I'm not sure how you think this was going to help. This line defines the program that is used to run your code. As you have Perl code, you need to run it with Perl. Trying to run it with bash is very unlikely to be useful.
The m? ... ? (or, formerly, ? ... ?) syntax triggers an obscure and specialised behaviour. It seems to me that this behaviour isn't required in your case, so you can probably change it to the more usual / ... /.
Your regex contains an unescaped dot character. Given that you seem to be extracting the basename from a filename that has an extension, it seems likely that this should be escaped (using \.) so that it matches an actual dot (rather than any character).
If you are using this code to extract a file's basename, then using a regex probably isn't the best approach. Perhaps take a look at File::Basename instead.
First, please note that I ask this question out of curiosity, and I'm aware that using variable names like ## is probably not a good idea.
When using doubles quotes (or qq operator), scalars and arrays are interpolated :
$v = 5;
say "$v"; # prints: 5
$# = 6;
say "$#"; # prints: 6
#a = (1,2);
say "#a"; # prints: 1 2
Yet, with array names of the form #+special char like ##, #!, #,, #%, #; etc, the array isn't interpolated :
#; = (1,2);
say "#;"; # prints nothing
say #; ; # prints: 1 2
So here is my question : does anyone knows why such arrays aren't interpolated? Is it documented anywhere?
I couldn't find any information or documentation about that. There are too many articles/posts on google (or SO) about the basics of interpolation, so maybe the answer was just hidden in one of them, or at the 10th page of results..
If you wonder why I could need variable names like those :
The -n (and -p for that matter) flag adds a semicolon ; at the end of the code (I'm not sure it works on every version of perl though). So I can make this program perl -nE 'push#a,1;say"#a"}{say#a' shorter by doing instead perl -nE 'push#;,1;say"#;"}{say#', because that last ; convert say# to say#;. Well, actually I can't do that because #; isn't interpolated in double quotes. It won't be useful every day of course, but in some golfing challenges, why not!
It can be useful to obfuscate some code. (whether obfuscation is useful or not is another debate!)
Unfortunately I can't tell you why, but this restriction comes from code in toke.c that goes back to perl 5.000 (1994!). My best guess is that it's because Perl doesn't use any built-in array punctuation variables (except for #- and #+, added in 5.6 (2000)).
The code in S_scan_const only interprets # as the start of an array if the following character is
a word character (e.g. #x, #_, #1), or
a : (e.g. #::foo), or
a ' (e.g. #'foo (this is the old syntax for ::)), or
a { (e.g. #{foo}), or
a $ (e.g. #$foo), or
a + or - (the arrays #+ and #-), but not in regexes.
As you can see, the only punctuation arrays that are supported are #- and #+, and even then not inside a regex. Initially no punctuation arrays were supported; #- and #+ were special-cased in 2000. (The exception in regex patterns was added to make /[\c#-\c_]/ work; it used to interpolate #- first.)
There is a workaround: Because #{ is treated as the start of an array variable, the syntax "#{;}" works (but that doesn't help your golf code because it makes the code longer).
Perl's documentation says that the result is "not strictly predictable".
The following, from perldoc perlop (Perl 5.22.1), refers to interpolation of scalars. I presume it applies equally to arrays.
Note also that the interpolation code needs to make a decision on
where the interpolated scalar ends. For instance, whether
"a $x -> {c}" really means:
"a " . $x . " -> {c}";
or:
"a " . $x -> {c};
Most of the time, the longest possible text that does not include
spaces between components and which contains matching braces or
brackets. because the outcome may be determined by voting based on
heuristic estimators, the result is not strictly predictable.
Fortunately, it's usually correct for ambiguous cases.
Some things are just because "Larry coded it that way". Or as I used to say in class, "It works the way you think, provided you think like Larry thinks", sometimes adding "and it's my job to teach you how Larry thinks."
I have a SQL query
select name from Employee
Output :
Sharma's
How can I store this output in perl string.
I tried below :
$sql =qq {select Name from Employee};
$Arr = &DataBaseQuery( $dbHandle, $sql );
$name = $Arr;
But when I print $name I get output as
Sharma::s
How can I store the single quote in the $name.
First of all, non of standard DBI/DBD exibits behavior you listed, in my experience.
Without knowing details of what DataBaseQuery() does it's impossible to answer conclusively, but a plausible theory can be formed:
Apostrophe is a valid package separator in Perl, equivalent to "::".
Reference: perldoc perlmod
The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and in part because it's more readable to emacs macros. It also makes C++ programmers feel like they know what's going on--as opposed to using the single quote as separator, which was there to make Ada programmers feel like they knew what was going on. Because the old-fashioned syntax is still supported for backwards compatibility, if you try to use a string like "This is $owner's house" , you'll be accessing $owner::s ; that is, the $s variable in package owner , which is probably not what you meant. Use braces to disambiguate, as in "This is ${owner}'s house" .
perl -e 'package A::B; $A::B=1; 1;
package main;
print "double-colon: $A::B\n";
print "apostrophe: $A'"'"'B\n";'
double-colon: 1
apostrophe: 1
I have a strong suspicion something within your own libraries inside DataBaseQuery() call was written to be "smart" and to convert apostrophes to double-colons because of this.
If you can't figure out root cause, you can always do one of the following:
Write your own DB wrapper
Assuming your text isn't likely to contain "::", run a regex to fix s#::#'#g; on all results from DataBaseQuery() (likely, in a function serving as a wrapper-replacement for DataBaseQuery())
I am trying to extract a part of a string and put it into a new variable. The string I am looking at is:
maker-scaffold_26653|ref0016423-snap-gene-0.1
(inside a $gene_name variable)
and the thing I want to match is:
scaffold_26653|ref0016423
I'm using the following piece of code:
my $gene_name;
my $scaffold_name;
if ($gene_name =~ m/scaffold_[0-9]+\|ref[0-9]+/) {
$scaffold_name = $1;
print "$scaffold_name\n";
}
I'm getting the following error when trying to execute:
Use of uninitialized value $scaffold_name in concatenation (.) or string
I know that the pattern is right, because if I use $' instead of $1 I get
-snap-gene-0.1
I'm at a bit of a loss: why will $1 not work here?
If you want to use a value from the matching you have to make () arround the character in regex
To expand on Jens' answer, () in a regex signifies an anonymous capture group. The content matched in a capture group is stored in $1-9+ from left to right, so for example,
/(..):(..):(..)/
on an HH:MM:SS time string will store hours, minutes, and seconds in $1, $2, $3 respectively. Naturally this begins to become unwieldy and is not self-documenting, so you can assign the results to a list instead:
my ($hours, $mins, $secs) = $time =~ m/(..):(..):(..)/;
So your example could bypass the use of $ variables by doing direct assignment:
my ($scaffold_name) = $gene_name =~ m/(scaffold_[0-9]+[|]ref[0-9]+)/;
# $scaffold_name now contains 'scaffold_26653|ref0016423'
You can even get rid of the ugly =~ binding by using for as a topicalizer:
my $scaffold_name;
for ($gene_name) {
($scaffold_name) = m/(scaffold_\d+[|]ref\d+)/;
print $scaffold_name;
}
If things start to get more complex, I prefer to use named capture groups (introduced in Perl v5.10.0):
$gene_name =~ m{
(?<scaffold_name> # ?<name> creates a named capture group
scaffold_\d+? # 'scaffold' and its trailing digits
[|] # Literal pipe symbol
ref\d+ # 'ref' and its trailing digits
)
}xms; # The x flag lets us write more readable regexes
print $+{scaffold_name}, "\n";
The results of named capture groups are stored in the magic hash %+. Access is done just like any other hash lookup, with the capture groups as the keys. %+ is locally scoped in the same way the $ are, so it can be used as a drop-in replacement for them in most situations.
It's overkill for this particular example, but as regexes start to get larger and more complicated, this saves you the trouble of either having to scroll all the way back up and count anonymous capture groups from left to right to find which of those darn $ variables is holding the capture you wanted, or scan across a long list assignment to find where to add a new variable to hold a capture that got inserted in the middle.
My personal rule of thumb is to assign the results of anonymous captured to descriptively named lexically scoped variables for 3 or less captures, then switch to using named captures, comments, and indentation in regexes when more are necessary.
The project I recently joined abstracts logic into code and database elements. Business logic like xPaths, regular expressions and function names are entered in the database, while general code like reading files, creating xml from xpaths, etc are in the code base.
Most (if not all) of the methods that use regular expressions are structured thus:
if ( $entry =~ /$regex/ ) { $req_value = $1; }
This means that only $1 is available and you always have to write your regex to give you your desired result in $1.
The issue:
The result for the following strings should be either
'2.6.9-78.1.6.ELsmp (SMP)' or '2.6.9-78.1.6.ELsmp'
depending on the existence of SMP. $1 does not suffice for $entry[0].
$entry[0] = qq|Linux version 2.6.9-78.1.6.ELsmp (brewbuilder#hs20-bc2-2.build.redhat.com) (gcc version 3.4.6 20060404 (Red Hat 3.4.6-10)) #1 SMP Wed Sep 24 05:41:12 EDT 2008|;
$entry[1] = qq|Linux version 2.6.9-78.0.5.ELsmp (brewbuilder#hs20-bc2-2.build.redhat.com) (gcc version 3.4.6 20060404 (Red Hat 3.4.6-10)) #1 Wed Sep 24 05:41:12 EDT 2008|;
Hence my solution:
my $mutable = '';
my $regex = qr/((\d.*?)\s+(?:.*)?(SMP)((?{$mutable="$2 ($3)"}))|(\d.*?))\s+/;
if ($entry[$i] =~ /$regex/) {
$req_value = $1;
$req_value = $mutable if ($mutable ne '');
$mutable = '';
}
Unfortunately, the existence of a 'variable' in the database makes this solution unacceptable.
My questions are:
How can I clean up the above solution to make it acceptable with the structure available?
or
How can I use a substitution regex with the structure 'if ($entry =~ /$regex/)'?
Thanks.
You're stuck unless you can talk the folks who control the code you're using into generalizing it somehow. The good news is you need only a bit more, perhaps
if (my #fields = $_ =~ /$pat/) {
$req_value = join " " => grep defined($_), #fields;
}
This works because a successful regular-expression match in list context returns all captured substrings, i.e., $1, $2, $3, and so on as appropriate.
With a single pattern,
qr/(\d+(?:[-.]\w+)*)(?:.*(SMP))?/
the code above yields 2.6.9-78.1.6.ELsmp SMP and 2.6.9-78.0.5.ELsmp in $req_value. The grep defined($_) filters out captures for subpatterns not taken. Without it, you get undefined value warnings for the non-SMP case.
The downside is every regular expression would need to be reviewed to be sure that all capturing groups really ought to go in $req_value. For example, say someone is using the pattern
qr/(XYZ) OS (version \d+|v-\d+)/
As it is now, only XYZ would go into $req_value, but using the above generalization would also include the version number. If that's undesired, the regular expression should be
qr/(XYZ) OS (?:version \d+|v-\d+)/
because (?:...) does not capture (that is, it does not produce a $2 for the pattern above): it's for grouping only.
I don't fully understand your constraints. Are you limited to supplying a single regex that will always by processed using the code in your first excerpt? If so, you cannot do what you are trying to do. You are trying to extract two separate parts of the entry string, you simply can't return 2 values in a single scalar return value unless you can add the code to concatenate them.
Can you add perl code at all? For example, can you define the logic to be:
if ( $entry =~ /$regex/ ) { $req_value = '$1 $2'; }
where your $regex = qr/((\d.*?)\s+(?:.*)?(SMP)/; ?
Baring the ability to define some new perl code, you can't accomplish this.
Regarding part two, substiutions. I interpret your question to ask if you can compile both the PATTERN and REPLACEMENT parts of s/PATTERN/REPLACEMENT/ into a single qr//. If so, you cannot. qr// only compiles a matching pattern, and a qr variable can only be used in the PATTERN portion of a REPLACEMENT. In other words, to use s///, you'll need to write perl code that runs s///. I'm guessing that if you could write new perl code, you'd use the above solution.
One more thought: In your current architecture, can you define fields in terms of of other fields? In other words, could you extract the version string with one regex, the SMP string with another regex, and define a third field that combines the two?
As of 5.10.0, (?|pattern) is available to allow alternatives to use the same capture numbering. As you pointed out that you're still using 5.8, this may not be useful directly but perhaps as further incentive to your project to start moving to a modern Perl.