I have some TSV data
ID Name Email
1 test test#email.com
321 stan stan#nowhere.net
I would like to parse this into a list of hashes
#entities[0]<Name> eq "test";
#entities[1]<Email> eq "stan#nowhere.net";
I'm having trouble with using the newline metacharacter to delimit the header row from the value rows. My grammar definition:
use v6;
grammar Parser {
token TOP { <headerRow><valueRow>+ }
token headerRow { [\s*<header>]+\n }
token header { \S+ }
token valueRow { [\s*<value>]+\n? }
token value { \S+ }
}
my $dat = q:to/EOF/;
ID Name Email
1 test test#email.com
321 stan stan#nowhere.net
EOF
say Parser.parse($dat);
But this is returning Nil. I think I'm misunderstanding something fundamental about regexes in raku.
Probably the main thing that's throwing it off is that \s matches horizontal and vertical space. To match just horizontal space, use \h, and to match just vertical space, \v.
One small recommendation I'd make is to avoid including the newlines in the token. You might also want to use the alternation operators % or %%, as they're designed for handling this type work:
grammar Parser {
token TOP {
<headerRow> \n
<valueRow>+ %% \n
}
token headerRow { <.ws>* %% <header> }
token valueRow { <.ws>* %% <value> }
token header { \S+ }
token value { \S+ }
token ws { \h* }
}
The result of Parser.parse($dat) for this is the following:
「ID Name Email
1 test test#email.com
321 stan stan#nowhere.net
」
headerRow => 「ID Name Email」
header => 「ID」
header => 「Name」
header => 「Email」
valueRow => 「 1 test test#email.com」
value => 「1」
value => 「test」
value => 「test#email.com」
valueRow => 「 321 stan stan#nowhere.net」
value => 「321」
value => 「stan」
value => 「stan#nowhere.net」
valueRow => 「」
which shows us that the grammar has successfully parsed everything. However, let's focus on the second part of your question, that you want to it to be available in a variable for you. To do that, you'll need to supply an actions class which is very simple for this project. You just make a class whose methods match the methods of your grammar (although very simple ones, like value/header that don't require special processing besides stringification, can be ignored). There are some more creative/compact ways to handle processing of yours, but I'll go with a fairly rudimentary approach for illustration. Here's our class:
class ParserActions {
method headerRow ($/) { ... }
method valueRow ($/) { ... }
method TOP ($/) { ... }
}
Each method has the signature ($/) which is the regex match variable. So now, let's ask what information we want from each token. In header row, we want each of the header values, in a row. So:
method headerRow ($/) {
my #headers = $<header>.map: *.Str
make #headers;
}
Any token with a quantifier on it will be treated as a Positional, so we could also access each individual header match with $<header>[0], $<header>[1], etc. But those are match objects, so we just quickly stringify them. The make command allows other tokens to access this special data that we've created.
Our value row will look identically, because the $<value> tokens are what we care about.
method valueRow ($/) {
my #values = $<value>.map: *.Str
make #values;
}
When we get to last method, we will want to create the array with hashes.
method TOP ($/) {
my #entries;
my #headers = $<headerRow>.made;
my #rows = $<valueRow>.map: *.made;
for #rows -> #values {
my %entry = flat #headers Z #values;
#entries.push: %entry;
}
make #entries;
}
Here you can see how we access the stuff we processed in headerRow() and valueRow(): You use the .made method. Because there are multiple valueRows, to get each of their made values, we need to do a map (this is a situation where I tend to write my grammar to have simply <header><data> in the grammar, and defeine the data as being multiple rows, but this is simple enough it's not too bad).
Now that we have the headers and rows in two arrays, it's simply a matter of making them an array of hashes, which we do in the for loop. The flat #x Z #y just intercolates the elements, and the hash assignment Does What We Mean, but there are other ways to get the array in hash you want.
Once you're done, you just make it, and then it will be available in the made of the parse:
say Parser.parse($dat, :actions(ParserActions)).made
-> [{Email => test#email.com, ID => 1, Name => test} {Email => stan#nowhere.net, ID => 321, Name => stan} {}]
It's fairly common to wrap these into a method, like
sub parse-tsv($tsv) {
return Parser.parse($tsv, :actions(ParserActions)).made
}
That way you can just say
my #entries = parse-tsv($dat);
say #entries[0]<Name>; # test
say #entries[1]<Email>; # stan#nowhere.net
TL;DR: you don't. Just use Text::CSV, which is able to deal with every format.
I will show how old Text::CSV will probably be useful:
use Text::CSV;
my $text = q:to/EOF/;
ID Name Email
1 test test#email.com
321 stan stan#nowhere.net
EOF
my #data = $text.lines.map: *.split(/\t/).list;
say #data.perl;
my $csv = csv( in => #data, key => "ID");
print $csv.perl;
The key part here is the data munging that converts the initial file into an array or arrays (in #data). It's only needed, however, because the csv command is not able to deal with strings; if data is in a file, you're good to go.
The last line will print:
${" 1" => ${:Email("test\#email.com"), :ID(" 1"), :Name("test")}, " 321" => ${:Email("stan\#nowhere.net"), :ID(" 321"), :Name("stan")}}%
The ID field will become the key to the hash, and the whole thing an array of hashes.
TL;DR regexs backtrack. tokens don't. That's why your pattern isn't matching. This answer focuses on explaining that, and how to trivially fix your grammar. However, you should probably rewrite it, or use an existing parser, which is what you should definitely do if you just want to parse TSV rather than learn about raku regexes.
A fundamental misunderstanding?
I think I'm misunderstanding something fundamental about regexes in raku.
(If you already know the term "regexes" is a highly ambiguous one, consider skipping this section.)
One fundamental thing you may be misunderstanding is the meaning of the word "regexes". Here are some popular meanings folk assume:
Formal regular expressions.
Perl regexes.
Perl Compatible Regular Expressions (PCRE).
Text pattern matching expressions called "regexes" that look like any of the above and do something similar.
None of these meanings are compatible with each other.
While Perl regexes are semantically a superset of formal regular expressions, they are far more useful in many ways but also more vulnerable to pathological backtracking.
While Perl Compatible Regular Expressions are compatible with Perl in the sense they were originally the same as standard Perl regexes in the late 1990s, and in the sense that Perl supports pluggable regex engines including the PCRE engine, PCRE regex syntax is not identical to the standard Perl regex used by default by Perl in 2020.
And while text pattern matching expressions called "regexes" generally do look somewhat like each other, and do all match text, there are dozens, perhaps hundreds, of variations in syntax, and even in semantics for the same syntax.
Raku text pattern matching expressions are typically called either "rules" or "regexes". The use of the term "regexes" conveys the fact that they look somewhat like other regexes (although the syntax has been cleaned up). The term "rules" conveys the fact they are part of a much broader set of features and tools that scale up to parsing (and beyond).
The quick fix
With the above fundamental aspect of the word "regexes" out of the way, I can now turn to the fundamental aspect of your "regex"'s behavior.
If we switch three of the patterns in your grammar for the token declarator to the regex declarator, your grammar works as you intended:
grammar Parser {
regex TOP { <headerRow><valueRow>+ }
regex headerRow { [\s*<header>]+\n }
token header { \S+ }
regex valueRow { [\s*<value>]+\n? }
token value { \S+ }
}
The sole difference between a token and a regex is that a regex backtracks whereas a token doesn't. Thus:
say 'ab' ~~ regex { [ \s* a ]+ b } # 「ab」
say 'ab' ~~ token { [ \s* a ]+ b } # 「ab」
say 'ab' ~~ regex { [ \s* \S ]+ b } # 「ab」
say 'ab' ~~ token { [ \s* \S ]+ b } # Nil
During processing of the last pattern (that could be and often is called a "regex", but whose actual declarator is token, not regex), the \S will swallow the 'b', just as it temporarily will have done during processing of the regex in the prior line. But, because the pattern is declared as a token, the rules engine (aka "regex engine") does not backtrack, so the overall match fails.
That's what's going on in your OP.
The right fix
A better solution in general is to wean yourself from assuming backtracking behavior, because it can be slow and even catastrophically slow (indistinguishable from the program hanging) when used in matching against a maliciously constructed string or one with an accidentally unfortunate combination of characters.
Sometimes regexs are appropriate. For example, if you're writing a one-off and a regex does the job, then you're done. That's fine. That's part of the reason that / ... / syntax in raku declares a backtracking pattern, just like regex. (Then again you can write / :r ... / if you want to switch on ratcheting -- "ratchet" means the opposite of "backtrack", so :r switches a regex to token semantics.)
Occasionally backtracking still has a role in a parsing context. For example, while the grammar for raku generally eschews backtracking, and instead has hundreds of rules and tokens, it nevertheless still has 3 regexs.
I've upvoted #user0721090601++'s answer because it's useful. It also addresses several things that immediately seemed to me to be idiomatically off in your code, and, importantly, sticks to tokens. It may well be the answer you prefer, which will be cool.
Related
A CSV style quoted string, for the purposes of this question, is a string in which:
The string starts and ends with exactly one ".
Two double quotes inside the string are collapsed to one double quote. "Alo""ha"→Alo"ha.
"" on its own is an empty string.
Error inputs, such as "A""" e", cannot be parsed. It's an A", followed by junk e".
I've tried several things, none of which have worked fully.
The closest I've gotten, thanks to some help from user pinkieval in #nom on the Mozilla IRC:
use std::error as stderror; /* Avoids needing nightly to compile */
named!(csv_style_string<&str, String>, map_res!(
terminated!(tag!("\""), not!(peek!(char!('"')))),
csv_string_to_string
));
fn csv_string_to_string(s: &str) -> Result<String, Box<stderror::Error>> {
Ok(s.to_string().replace("\"\"", "\""))
}
This does not catch the end of the string correctly.
I've also attempted to use the re_match! macro with r#""([^"]|"")*""#, but that always results in an Err::Incomplete(1).
I've determined that the given CSV example for Nom 1.0 doesn't work for a quoted CSV string as I'm describing it, but I do know implementations differ.
Here is one way of doing it:
use nom::types::CompleteStr;
use nom::*;
named!(csv_style_string<CompleteStr, String>,
delimited!(
char!('"'),
map!(
many0!(
alt!(
// Eat a " delimiter and the " that follows it
tag!("\"\"") => { |_| '"' }
| // Normal character
none_of!("\"")
)
),
// Make a string from a vector of chars
|v| v.iter().collect::<String>()
),
char!('"')
)
);
fn main() {
println!(r#""Alo\"ha" = {:?}"#, csv_style_string(CompleteStr(r#""Alo""ha""#)));
println!(r#""" = {:?}"#, csv_style_string(CompleteStr(r#""""#)));
println!(r#"bad format: {:?}"#, csv_style_string(CompleteStr(r#""A""" e""#)));
}
(I wrote it in full nom, but a solution like yours, based on an external function instead of map!() each character, would work too, and may be more efficient.)
The magic here, that would also solve your regexp issue, is to use CompleteStr. This basically tells nom that nothing will come after that input (otherwise, nom assumes you're doing a streaming parser, so more input may follow).
This is needed because we need to know what to do with a " if it is the last character fed to nom. Depending on the character that comes after it (another ", a normal character, or EOF), we have to take a different decision -- hence the Incomplete result, meaning nom does not have enough input to make the decision. Telling nom that EOF comes next solves this indecision.
Further reading on Incomplete on nom's author's blog: http://unhandledexpression.com/general/2018/05/14/nom-4-0-faster-safer-simpler-parsers.html#dealing-with-incomplete-usage
You may note that this parser does not actually rejects the invalid input, but parses the beginning and returns the rest. If you use this parser as a subparser in another parser, the latter would then feed the remainder to the next subparser, which would crash as well (because it would expect a comma), causing the overall parser to fail.
If you don't want that, you could make csv_style_string match peek!(alt!(char!(',')|char!('\n")|eof!())).
I have a request that I make in an API using GET
LWP::UserAgent,
the data is returned as JSON, with up to two results at most as follows:
{
"status":1,
"time":1507891855,
"response":{
"prices":{
"nome1\u2122":{
"preco1":1111,
"preco2":1585,
"preco3":1099
},
"nome2":{
"preco1":519,
"preco2":731,
"preco3":491
}
}
}
}
Dump:
$VAR1 = {
'status' => 1,
'time' => 1507891855,
'response' => {
'prices' => {
'nome1' => {
'preco1' => 1111,
'preco3' => 1099,
'preco2' => 1585
},
'nome2' => {
'preco3' => 491,
'preco1' => 519,
'preco2' => 731
}
}
}
};
What I would like to do is:
Take this data and save it in a variable to make a comparison using if with another variable that already has the name stored. The comparison would be with name1 / name2 and if it is true with the other variable it would get preco2 and preco3 to print everything
My biggest problem in the case is that some of these names in JSON contain characters like (TradeMark) that comes as \u2122 (some cases are other characters), so I can not make the comparison with the name of the other variable that is already with the correct name
nome1™
If I could only save the JSON already "converted" the characters would help me with the rest.
Basically after doing the request for the API I want to save the contents in a variable already converting all \u2122 to their respective character (this is the part that I do not know how to do in Perl) and then using another variable to compare them names are equal to show the price
Thanks for the help and any questions please tell me that I try to explain again in another way.
If I understand correctly, you need to get the JSON that you receive in UTF8 format to an internal variable that you can process. For that, you may use JSON::XS:
use utf8;
use JSON::XS;
my $name = "nome1™";
my $var1 = decode_json $utf8_encoded_json_text;
# Compare with name in $name
if( defined $var1->{'response'}->{'prices'}->{$name} ) {
# Do something with the name that matches
my $match = $var1->{'response'}->{'prices'}->{$name};
print $match->{'preco1'}, "\n";
}
Make sure you tell the Perl interpreter that your source is in UTF8 by specifying use utf8; at the beginning of the script. Then make sure you are editing the script with an editor that supports that format.
The function decode_json will return a ref to the converted value. In this case a hash ref. From there you work your way into the JSON.
If you know $name is going to be in the JSON you may omit the defined part. Otherwise, the defined clause will tell you whether the hash value is there. One you know, you may do something with it. If the hash values are a single word with no special characters, you may use $var1->{response}->{prices}->{$name}, but it is always safer to use $var1->{'response'}->{'prices'}->{$name}. Perl gets a bit ugly handling hash refs...
By the way, in JSON::XS you will also find the encode_json function to do the opposite and also an object oriented interface.
I got three different entries "10576.53012.46344.35174" , "10" and "Doc-15" in foreach loop. Out of these 3 entries, i want 10576.53012.46344.35174. How can i verify that current string contains multiple . and numbers.
Im new to TCL, Need suggestion
This is the sort of task that is a pretty good fit for regular expressions.
The string 10576.53012.46344.35174 is matched by a RE like this: ^\d{5}(?:\.\d{5}){3}$ though you might want something a little less strict (e.g., with more flexibility in the number of digits per group — 5 — or the number of groups following a . — 3).
You test if a string matches a regular expression with the regexp command:
if {[regexp {^\d{5}(?:\.\d{5}){3}$} $theVarWithTheString]} {
puts "the regular expression matched $theVarWithTheString"
}
An alternative approach is to split the string by . and check that each group is what you want:
set goodBits 0
set badString 0
foreach group [split $theVarWithTheString "."] {
if {![string is integer -strict $group]} {
set badString 1
break
}
incr goodBits
}
if {!$badString && $goodBits == 4} {
puts "the string was OK"
}
I greatly prefer the regular expression approach myself (with occasional help from string is as appropriate). Writing non-RE validators can be challenging and tends to require a lot of code.
I am confused about accessing the contents of some JSON data that I have decoded. Here is an example
I don't understand why this solution works and my own does not. My questions are rephrased below
my $json_raw = getJSON();
my $content = decode_json($json_raw);
print Data::Dumper($content);
At this point my JSON data has been transformed into this
$VAR1 = { 'items' => [ 1, 2, 3, 4 ] };
My guess tells me that, once decoded, the object will be a hash with one element that has the key items and an array reference as the value.
$content{'items'}[0]
where $content{'items'} would obtain the array reference, and the outer $...[0] would access the first element in the array and interpret it as a scalar. However this does not work. I get an error message use of uninitialized value [...]
However, the following does work:
$content->{items}[0]
where $content->{items} yields the array reference and [0] accesses the first element of that array.
Questions
Why does $content{'items'} not return an array reference? I even tried #{content{'items'}}, thinking that, once I got the value from content{'items'}, it would need to be interpreted as an array. But still, I receive the uninitialized array reference.
How can I access the array reference without using the arrow operator?
Beginner's answer to beginner :) Sure not as profesional as should be, but maybe helps you.
use strict; #use this all times
use warnings; #this too - helps a lot!
use JSON;
my $json_str = ' { "items" : [ 1, 2, 3, 4 ] } ';
my $content = decode_json($json_str);
You wrote:
My guess tells me that, once decoded, the object will be a hash with
one element that has the key items and an array reference as the value.
Yes, it is a hash, but the the decode_json returns a reference, in this case, the reference to hash. (from the docs)
expects an UTF-8 (binary) string and tries to parse that
as an UTF-8 encoded JSON text,
returning the resulting reference.
In the line
my $content = decode_json($json_str);
you assigning to an SCALAR variable (not to hash).
Because you know: it is a reference, you can do the next:
printf "reftype:%s\n", ref($content);
#print: reftype:HASH ^
#therefore the +------- is a SCALAR value containing a reference to hash
It is a hashref - you can dump all keys
print "key: $_\n" for keys %{$content}; #or in short %$content
#prints: key: items
also you can assing the value of the "items" (arrayref) to an scalar variable
my $aref = $content->{items}; #$hashref->{key}
#or
#my $aref = ${$content}{items}; #$hash{key}
but NOT
#my $aref = $content{items}; #throws error if "use strict;"
#Global symbol "%content" requires explicit package name at script.pl line 20.
The $content{item} is requesting a value from the hash %content and you never defined/assigned such variable. the $content is an scalar variable not hash variable %content.
{
#in perl 5.20 you can also
use 5.020;
use experimental 'postderef';
print "key-postderef: $_\n" for keys $content->%*;
}
Now step deeper - to the arrayref - again you can print out the reference type
printf "reftype:%s\n", ref($aref);
#reftype:ARRAY
print all elements of array
print "arr-item: $_\n" for #{$aref};
but again NOT
#print "$_\n" for #aref;
#dies: Global symbol "#aref" requires explicit package name at script.pl line 37.
{
#in perl 5.20 you can also
use 5.020;
use experimental 'postderef';
print "aref-postderef: $_\n" for $aref->#*;
}
Here is an simple rule:
my #arr; #array variable
my $arr_ref = \#arr; #scalar - containing a reference to #arr
#{$arr_ref} is the same as #arr
^^^^^^^^^^ - array reference in curly brackets
If you have an $arrayref - use the #{$array_ref} everywhere you want use the array.
my %hash; #hash variable
my $hash_ref = \%hash; #scalar - containing a reference to %hash
%{$hash_ref} is the same as %hash
^^^^^^^^^^^ - hash reference in curly brackets
If you have an $hash_ref - use the %{$hash_ref} everywhere you want use the hash.
For the whole structure, the following
say $content->{items}->[0];
say $content->{items}[0];
say ${$content}{items}->[0];
say ${$content}{items}[0];
say ${$content->{items}}[0];
say ${${$content}{items}}[0];
prints the same value 1.
$content is a hash reference, so you always need to use an arrow to access its contents. $content{items} would refer to a %content hash, which you don't have. That's where you're getting that "use of uninitialized value" error from.
I actually asked a similar question here
The answer:
In Perl, a function can only really return a scalar or a list.
Since hashes can be initialized or assigned from lists (e.g. %foo = (a => 1, b => 2)), I guess you're asking why json_decode returns something like { a => 1, b => 2 } (a reference to an anonymous hash) rather than (a => 1, b => 2) (a list that can be copied into a hash).
I can think of a few good reasons for this:
in Perl, an array or hash always contains scalars. So in something like { "a": { "b": 3 } }, the { "b": 3 } part has to be a scalar; and for consistency, it makes sense for the whole thing to be a scalar in the same way.
if the hash is quite large (many keys at top-level), it's pointless and expensive to iterate over all the elements to convert it into a list, and then build a new hash from that list.
in JSON, the top-level element can be either an object (= Perl hash) or an array (= Perl array). If json_decode returned a list in the former case, it's not clear what it would return in the latter case. After decoding the JSON string, how could you examine the result to know what to do with it? (And it wouldn't be safe to write %foo = json_decode(...) unless you already knew that you had a hash.) So json_decode's behavior works better for any general-purpose library code that has to use it without already knowing very much about the data it's working with.
I have to wonder exactly what you passed as an array to json_decode, because my results differ from yours.
#!/usr/bin/perl
use JSON qw (decode_json);
use Data::Dumper;
my $json = '["1", "2", "3", "4"]';
my $fromJSON = decode_json($json);
print Dumper($fromJSON);
The result is $VAR1 = [ '1', '2', '3', '4' ];
Which is an array ref, where your result is a hash ref
So did you pass in a hash with element items which was a reference to an array?
In my example you would get the array by doing
my #array = #{ $fromJSON };
In yours
my #array = #{ $content->{'items'} }
I don't understand why you dislike the arrow operator so much!
The decode_json function from the JSON module will always return a data reference.
Suppose you have a Perl program like this
use strict;
use warnings;
use JSON;
my $json_data = '{ "items": [ 1, 2, 3, 4 ] }';
my $content = decode_json($json_data);
use Data::Dump;
dd $content;
which outputs this text
{ items => [1 .. 4] }
showing that $content is a hash reference. Then you can access the array reference, as you found, with
dd $content->{items};
which shows
[1 .. 4]
and you can print the first element of the array by writing
print $content->{items}[0], "\n";
which, again as you have found, shows just
1
which is the first element of the array.
As #cjm mentions in a comment, it is imperative that you use strict and use warnings at the start of every Perl program. If you had those in place in the program where you tried to access $content{items}, your program would have failed to compile, and you would have seen the message
Global symbol "%content" requires explicit package name
which is a (poorly-phrased) way of telling you that there is no %content so there can be no items element.
The scalar variable $content is completely independent from the hash variable %content, which you are trying to access when you write $content{items}. %content has never been mentioned before and it is empty, so there is no items element. If you had tried #{$content->{items}} then it would have worked, as would #{${$content}{items}}
If you really have a problem with the arrow operator, then you could write
print ${$content}{items}[0], "\n";
which produces the same output; but I don't understand what is wrong with the original version.
I'm trying to construct json text as show below. But the variables such as $token, $state, $failedServers are not been replaced with its value. Note- I don't want to use any module specifically for this to work, I just want some plain string to work. Can anyone help me ?
my $json = '{"serverToken":"$token", "state":"$state","parameters" :"$failedServers"}';
current output was:
{"serverToken":"$token", "state":"$state","parameters" :"$failedServers"}
needed output format:
{"serverToken":"1213", "state":"failed","parameters" :"oracleapps.veeralab.com,suntrust.com"}
Your variables are not being replaced, because they are inside of a single-quoted string--that is, they are inside a string quoted by ' characters. This prevents variable substitution.
You will also be much better off creating JSON using a JSON library, such as this one. Simply using a quoted string is very dangerous. Suppose your one of your variables ends up containing a special character; you will end up with invalid JSON.
{"serverToken":"123"ABC", "state":"offline", "paramameters":"bugs"}
If your variables come from user input, really bad things could happen. Imagine that $token is set to equal foo", "state":"online", "foo":"bar. Your resulting JSON structure would be:
{"serverToken":"foo", "state":"online", "foo":"bar", "state":"offline" ...
Certainly not what you want.
Possible solutions:
The most blatantly obvious solution is simply not to the ' quote character. This has the drawback of requiring you to escape your double quote (") characters, though, but it's easy:
my $json = "{\"serverToken\":\"$token\", \"state\":\"$state\",\"parameters\" :\"$failedServers\"}";
Another option is to use sprintf:
my $json = sprintf('{"serverToken":"%s", "state":"%s", "parameters":"%s"}', $token, $state, $failedServers);
But by far, the best solution, because it won't break with wonky input, is to use a library:
use JSON;
my $json = encode_json( {
serverToken => $token,
state => $state,
paramaters => $failedServers
} );