My RAKU Code:
sub comments {
if ($DEBUG) { say "<filtering comments>\n"; }
my #filteredtitles = ();
# This loops through each track
for #tracks -> $title {
##########################
# LAB 1 TASK 2 #
##########################
## Add regex substitutions to remove superflous comments and all that follows them
## Assign to $_ with smartmatcher (~~)
##########################
$_ = $title;
if ($_) ~~ s:g:mrx/ .*<?[\(^.*]> / {
# Repeat for the other symbols
########################## End Task 2
# Add the edited $title to the new array of titles
#filteredtitles.push: $_;
}
}
# Updates #tracks
return #filteredtitles;
}
Result when compiling:
Error Compiling! Placeholder variable '#_' may not be used here because the surrounding block doesn't take a signature.
Is there something obvious that I am missing? Any help is appreciated.
So, in contrast with #raiph's answer, here's what I have:
my #tracks = <Foo Ba(r B^az>.map: { S:g / <[\(^]> // };
Just that. Nothing else. Let's dissect it, from the inside out:
This part: / <[\(^]> / is a regular expression that will match one character, as long as it is an open parenthesis (represented by the \() or a caret (^). When they go inside the angle brackets/square brackets combo, it means that is an Enumerated character class.
Then, the: S introduces the non-destructive substitution, i.e., a quoting construct that will make regex-based substitutions over the topic variable $_ but will not modify it, just return its value with the modifications requested. In the code above, S:g brings the adverb :g or :global (see the global adverb in the adverbs section of the documentation) to play, meaning (in the case of the substitution) "please make as many as possible of this substitution" and the final / marks the end of the substitution text, and as it is adjacent to the second /, that means that
S:g / <[\(^]> //
means "please return the contents of $_, but modified in such a way that all its characters matching the regex <[\(^]> are deleted (substituted for the empty string)"
At this point, I should emphasize that regular expressions in Raku are really powerful, and that reading the entire page (and probably the best practices and gotchas page too) is a good idea.
Next, the: .map method, documented here, will be applied to any Iterable (List, Array and all their alikes) and will return a sequence based on each element of the Iterable, altered by a Code passed to it. So, something like:
#x.map({ S:g / foo /bar/ })
essencially means "please return a Sequence of every item on #x, modified by substituting any appearance of the substring foo for bar" (nothing will be altered on #x). A nice place to start to learn about sequences and iterables would be here.
Finally, my one-liner
my #tracks = <Foo Ba(r B^az>.map: { S:g / <[\(^]> // };
can be translated as:
I have a List with three string elements
Foo
Ba(r
B^az
(This would be a placeholder for your "list of titles"). Take that list and generate a second one, that contains every element on it, but with all instances of the chars "open parenthesis" and "caret" removed.
Ah, and store the result in the variable #tracks (that has my scope)
Here's what I ended up with:
my #tracks = <Foo Ba(r B^az>;
sub comments {
my #filteredtitles;
for #tracks -> $_ is copy {
s:g / <[\(^]> //;
#filteredtitles.push: $_;
}
return #filteredtitles;
}
The is copy ensures the variable set up by the for loop is mutable.
The s:g/...//; is all that's needed to strip the unwanted characters.
One thing no one can help you with is the error you reported. I currently think you just got confused.
Here's an example of code that generates that error:
do { #_ }
But there is no way the code you've shared could generate that error because it requires that there is an #_ variable in your code, and there isn't one.
One way I can help in relation to future problems you may report on StackOverflow is to encourage you to read and apply the guidance in Minimal Reproducible Example.
While your code did not generate the error you reported, it will perhaps help you if you know about some of the other compile time and run time errors there were in the code you shared.
Compile-time errors:
You wrote s:g:mrx. That's invalid: Adverb mrx not allowed on substitution.
You missed out the third slash of the s///. That causes mayhem (see below).
There were several run-time errors, once I got past the compile-time errors. I'll discuss just one, the regex:
.*<?[...]> will match any sub-string with a final character that's one of the ones listed in the [...], and will then capture that sub-string except without the final character. In the context of an s:g/...// substitution this will strip ordinary characters (captured by the .*) but leave the special characters.
This makes no sense.
So I dropped the .*, and also the ? from the special character pattern, changing it from <?[...]> (which just tries to match against the character, but does not capture it if it succeeds) to just <[...]> (which also tries to match against the character, but, if it succeeds, does capture it as well).
A final comment is about an error you made that may well have seriously confused you.
In a nutshell, the s/// construct must have three slashes.
In your question you had code of the form s/.../ (or s:g/.../ etc), without the final slash. If you try to compile such code the parser gets utterly confused because it will think you're just writing a long replacement string.
For example, if you wrote this code:
if s/foo/ { say 'foo' }
if m/bar/ { say 'bar' }
it'd be as if you'd written:
if s/foo/ { say 'foo' }\nif m/...
which in turn would mean you'd get the compile-time error:
Missing block
------> if m/⏏bar/ { ... }
expecting any of:
block or pointy block
...
because Raku(do) would have interpreted the part between the second and third /s as the replacement double quoted string of what it interpreted as an s/.../.../ construct, leading it to barf when it encountered bar.
So, to recap, the s/// construct requires three slashes, not two.
(I'm ignoring syntactic variants of the construct such as, say, s [...] = '...'.)
Related
From the docs (perldoc -f m)
If ? is the delimiter, then a match-only-once rule applies, described in m?*PATTERN*? below.
The "match-only-once rule" doesn't' seem to be defined anywhere, but it seems to be a real optimization,
use Benchmark qw(:all) ;
use constant HAYSTACK => "this is a test string";
my $needle = "test";
cmpthese(-1, {
'questionmark' => sub { if ( HAYSTACK =~ m?$needle?n ) { 1 } },
'backslash' => sub { if ( HAYSTACK =~ m/$needle/n ) { 1 } },
});
With the results,
Rate backslash questionmark
backslash 9267717/s -- -57%
questionmark 21588328/s 133% --
This makes me wonder why is the behavior in m// in scalar context such that it even needs this behavior? Let's take for example the output
perl -E'say "FOOOOOO" =~ m/O/' # returns 1
If it's not even counting the O what does it do after the first match such that it's twice as slow?
The "match-only-once rule" doesn't' seem to be defined anywhere, […]
"A match-only-once rule" is a description of the rule — it's a rule saying that m?PATTERN? matches only once — not an official name that you can use to search. The text that you quote is pulled from the perlop manpage, so when it says "described in m?*PATTERN*? below", it's referring to this part of that manpage:
m?PATTERN?msixpodualngc
This is just like the m/PATTERN/ search, except that it matches only once between calls to the reset() operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only m?? patterns local to the current package are reset.
while (<>) {
if (m?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear m?? status for next file
}
Another example switched the first "latin1" encoding it finds to "utf8" in a pod file:
s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
This makes me wonder why is the behavior in m// in scalar context such that it even needs this behavior?
Even in scalar context, m// or m?? may be called many times between resets, and if so then the two behave differently. (You can see this in the first snippet above. It's also the reason that your benchmarks give different performance results: the version with m?$needle?n only does a regex match the first time the function is called — it just returns 'no match' on all subsequent calls — whereas the version with m/$needle/n does a regex match every time.)
The confusion here is that "once" in "match-only-once" is in reference to the calling context of the m?? not in reference to matching once the needle inside the haystack, and ignoring subsequent matches of the needle inside the haystack. So if m?? is called many times without reset, only the first one that matches will return the match.
sub foo { return "foo" =~ m?o? };
say foo(); # 1
say foo(); # undef
reset();
say foo(); # 1
I am downloading a webpage and converting into a string using LWP::Simple. When I copy the results into an editor I find multiple instances of the pattern I'm looking for "data-src-hq".
While I'm trying to do something more complex using regex I am starting in baby steps so I can properly learn how to use regex, I started off with just to match "data-src-hq" with the following code:
if($html =~ /data-src-hq/ism)
{
print "match\n";
}
else
{
print "nope\n";
}
My code returns "nope". However, if I modify the pattern search to just "data" or "data-src" I do get a match. The same happens no matter how I use and combine the string and multiline modifier.
My understanding is that a hyphen is not a special character unless it's within brackets, am I missing something simple?
How to fix this?
You are likely getting two outputs, one of match and one of nope. Your code is missing the keyword else:
See your code's current execution here
if($html =~ /data-src-hq/ism)
{
print "match\n";
}
{
print "nope\n";
}
Should be:
See this code's execution here
if($html =~ /data-src-hq/ism)
{
print "match\n";
}
else {
print "nope\n";
}
Otherwise, your code is fine and works to identify whether data-src-hq exists in $html.
So why does your existing code output nope?
That's because {} is a basic block (see Basic BLOCKs in Perl's documentation). An excerpt from the documentation:
A BLOCK by itself (labeled or not) is semantically equivalent to a
loop that executes once. Thus you can use any of the loop control
statements in it to leave or restart the block. (Note that this is NOT
true in eval{}, sub{}, or contrary to popular belief do{} blocks,
which do NOT count as loops.) The continue block is optional.
I have text files containing lines like:
2/17/2018 400000098627 =2,000.0 $2.0994 $4,387.75
3/7/2018 1)0000006043 2,000.0 $2.0731 $4,332.78
3/26/2018 4 )0000034242 2,000.0 $2.1729 $4,541.36
4/17/2018 2)0000008516 2,000.0 $2.219 $4,637.71
I am matching them with /^\s*(\S+)\s+(?:[0-9|\)| ]+)+\s+([0-9|.|,]+)\s+\$/ But I also have some files with lines in a completely different format, which I match with a different regex. When I open a file I determine which format and assign $pat = '<regex-string>'; in a switch/case block:
$pat = '/^\s*(\S+)\s+(?:[0-9|\)| ]+)+\s+([0-9|.|,]+)\s+\$/'
But the ? character that introduces the non-capturing group I use to match repeats after the date and before the first currency amount causes the Perl interpreter to fail to compile the script, reporting on abort:
syntax error at ./report-dates-amounts line 28, near "}continue "
If I delete the ? character, or replace ? with \? escaped character, or first assign $q = '?' then replace ? with $q inside a " string assignment (ie. $pat = "/^\s*(\S+)\s+($q:[0-9|\)| ]+)+\s+([0-9|.|,]+)\s+\$/"; ) the script compiles and runs. If I assign the regex string outside the switch/case block that also works OK. Perl v5.26.1 .
My code also doesn't have any }continue in it, which as reported in the compilation failure is probably some kind of transformation of the switch/case code by Switch.pm into something native the compiler chokes on. Is this some kind of bug in Switch.pm? It fails even when I use given/when in exactly the same way.
#!/usr/local/bin/perl
use Switch;
# Edited for demo
switch($format)
{
# Format A eg:
# 2/17/2018 400000098627 =2,000.0 $2.0994 $4,387.75
# 3/7/2018 1)0000006043 2,000.0 $2.0731 $4,332.78
# 3/26/2018 4 )0000034242 2,000.0 $2.1729 $4,541.36
# 4/17/2018 2)0000008516 2,000.0 $2.219 $4,637.71
#
case /^(?:april|snow)$/i
{ # This is where the ? character breaks compilation:
$pat = '^\s*(\S+)\s+(?:[0-9|\)| ]+)+\s+\D?(\S+)\s+\$';
# WORKS:
# $pat = '^\s*(\S+)\s+(' .$q. ':[0-9|\)| ]+)+\s+\D' .$q. '(\S+)\s+\$';
}
# Format B
case /^(?:umberto|petro)$/i
{
$pat = '^(\S+)\s+.*Think 1\s+(\S+)\s+';
}
}
Don't use Switch. As mentionned by #choroba in the comments, Switch uses a source filter, which leads to mysterious and hard to debug errors, as you constated.
The module's documentation itself says:
In general, use given/when instead. It were introduced in perl 5.10.0. Perl 5.10.0 was released in 2007.
However, given/when is not necessarily a good option as it is experimental and likely to change in the future (it seems that this feature was almost removed from Perl v5.28; so you definitely don't want to start using it now if you can avoid it). A good alternative is to use for:
for ($format) {
if (/^(?:april|snow)$/i) {
...
}
elsif (/^(?:umberto|petro)$/i) {
...
}
}
It might look weird a first, but once you get used to it, it's actually reasonable in my opinion. Or, of course, you can use none of this options and just do:
sub pattern_from_format {
my $format = shift;
if ($format =~ /^(?:april|snow)$/i) {
return qr/^\s*(\S+)\s+(?:[0-9|\)| ]+)+\s+\D?(\S+)\s+\$/;
}
elsif ($format =~ /^(?:umberto|petro)$/i) {
return qr/^(\S+)\s+.*Think 1\s+(\S+)\s+/;
}
# Some error handling here maybe
}
If, for some reason, you still want to use Switch: use m/.../ instead of /.../.
I have no idea why this bug is happening, however, the documentation says:
Also, the presence of regexes specified with raw ?...? delimiters may cause mysterious errors. The workaround is to use m?...? instead.
Which I misread at first, and therefore tried to use m/../ instead of /../, which fixed the issue.
Another option instead of an if/elsif chain would be to loop over a hash which maps your regular expressions to the values which should be assigned to $pat:
#!/usr/local/bin/perl
my %switch = (
'^(?:april|snow)$' => '^\s*(\S+)\s+(?:[0-9|\)| ]+)+\s+\D?(\S+)\s+\$',
'^(?:umberto|petro)$' => '^(\S+)\s+.*Think 1\s+(\S+)\s+',
);
for my $re (keys %switch) {
if ($format =~ /$re/i) {
$pat = $switch{$re};
last;
}
}
For a more general case (i.e., if you're doing more than just assigning a string to a scalar) you could use the same general technique, but use coderefs as the values of your hash, thus allowing it to execute an arbitrary sub based on the match.
This approach can cover a pretty wide range of the functionality usually associated with switch/case constructs, but note that, because the conditions are pulled from the keys of a hash, they'll be evaluated in a random order. If you have data which could match more than one condition, you'll need to take extra precautions to handle that, such as having a parallel array with the conditions in the proper order or using Tie::IxHash instead of a regular hash.
I have some doubt about the outcome of a binding operator expression in perl. I mean expression like
string =~ /pattern/
I have done some simple test
$ss="a1b2c3";
say $ss=~/a/; # 1
say $ss=~/[a-z]/g; # abc
#aa=$ss=~/[a-z]/g;say #aa; # abc
$aa=#aa;say $aa; # 3
$aa=$ss=~/[a-z]/g;say $aa; # 1
note the comment part above is the running result.
So here comes the question, what on earth is returned by $ss=~/[a-z]/g, it seems that it returned an array according to code line 3,4,5. But what about the last line, why it gives 1 instead of 3 which is the length of array?
The return of the match operator depends on the context: in list context it returns all captured matches, in scalar context the true/false. The say imposes list context, but in the first example nothing is captured in the regex so you only get "success."
Next, the behavior of /g modifier also differs across contexts. In list context, with it the string keeps being scanned with the given pattern until all matches are found, and a list with them is returned. These are your second and third examples.
But in scalar context its behavior is a bit specific: with it the search will continue from the position of the last match, the next time round. One typical use is in the loop condition
while (/(\w+)/g) { ... }
This is a bit of a tokenizer: after the body of the loop runs the next word is found, etc.
Then the last example doesn't really make sense; you are getting the "normal" scalar-context matching success/fail, and /g doesn't do anything -- until you match on $ss the next time
perl -wE'
$s=shift||q(abc);
for (1..2) { $m = $s=~/(.)/g; say "$m: $1"; }
'
prints lines 1:a and then 1:b.
Outside of iterative structures (like while condition) the /g in scalar context is usually an error, pointless at best or a quiet bug.
See "Global matching" under "Using regular expressions" in perlretut for /g.
See regex operators in perlop in general, and about /g as well. A useful tool to explore /g workings is pos.
I am pretty new to Perl. I have the following code fragment that works just fine, but I don't fully understand it:
for ($i = 1; $i <= $pop->Count(); $i++) {
foreach ( $pop->Head( $i ) ) {
/^(From|Subject):\s+/i and print $_, "\n";
}
}
$pop->Head is a string or an array of strings returned by the function Mail::POP3Client, and it is the headers of a bunch of emails. Line 3 is some kind of regular expression that extracts the FROM and the SUBJECT from the header.
My question is how does the print function only print the From and the Subject without all the other stuff in the header? What does "and" mean - this surely can't be a boolean and can it? Most important, I want to put the From string into its own variable (my $fromline). How do I do this?
I am hoping that this will be easy for some Perl professional, it has got me baffled!
Thanks in advance.
ARGHHH... The question was edited while I was typing the answer. OK, throwing out the part of my answer that's no longer relevant, and focusing on the specific questions:
The outer loop iterates over all the messages in the mailbox.
The inner loop doesn't specify a loop variable, so the special variable $_ is used.
In each iteration through the inner loop, $_ is one header line from message number $i.
/^(From|Subject):\s+/i and print $_, "\n";
The first part of this line, up to the and is a pattern. We didn't specify what to do with the pattern, so it's implicitly matched against $_. (That's one of the things that makes $_ special.) This gives us a yes/no test: does the pattern match the header line or not?
The pattern tests whether that item begins with (<) either of the words "From" or "Subject", followed immediately by a colon and one or more whitespace characters. (This not the correct pattern to match an RFC 822 header. Whitespace is optional on both sides of the colon. The pattern should more properly be /^(From|Subject)\s*:\s*/i. But that's a separate issue.) the i at the end of the pattern says to ignore case, so from or SUBJECT would be OK.
The and says to continue evaluating (i.e., executing) the expression if there is a match. If there's no match, whatever follows and is ignored.
The rest of the expression prints the header line ($_) and a newline ("\n").
In perl, and and or are boolean operators. They're synonyms for && and ||, except that they have much lower precedence, making it easier to write short-ciruit expressions without clutter from lots of parentheses.
The smallest change that captures the From line into a separate variable would be to add the following line to the inner loop:
/^From\s*:\s*(.*)$/i and $fromline = $1;
You should probably also put
$fromline = undef
before the loop so you can test, after the loop, whether there was a From: line.
There are other ways to do it. In fact, that's one of the mantras of perl: "There's more than one way to do it." I've stripped out the "From: " from the beginning of the line before storing the balance in $fromline, but I don't know your needs.
It's a logical and with short-circuiting. If the left side evaluates to true -- say, if that regular expression matches -- it'll evaluate the right side, the print.
If the expression on the left is false, it doesn't need to evaluate the right hand side, because the net result would still be false, so it skips it.
See also: perldoc perlop