regexp/syntax: cleanup code generation in perl_groups.go

Cleanup code generation of perl_groups.go:
* Fix the generated code header to follow the standard https://go.dev/s/generatedcode
* Apply gofmt as last step of code generation
* Add //go:generate lines in parse.go to trigger code generation
* Adapt make_perl_groups.pl to handle writing directly to the output
  file (as we can't use shell redirection in go:generate lines)
* use strict; use warnings;

Change-Id: I675241da03dd3f6facc9eb9de00999bd9203ad70
Reviewed-on: https://go-review.googlesource.com/c/go/+/555995
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/regexp/syntax/make_perl_groups.pl

@@ -11,7 +11,10 @@
 # Perl about each letter from 0-128 and write down
 # its answer.
 
-@posixclasses = (
+use strict;
+use warnings;
+
+my @posixclasses = (
 	"[:alnum:]",
 	"[:alpha:]",
 	"[:ascii:]",
@@ -28,13 +31,13 @@
 	"[:xdigit:]",
 );
 
-@perlclasses = (
+my @perlclasses = (
 	"\\d",
 	"\\s",
 	"\\w",
 );
 
-%overrides = (
+my %overrides = (
 	# Prior to Perl 5.18, \s did not match vertical tab.
 	# RE2 preserves that original behaviour.
 	"\\s:11" => 0,
@@ -70,7 +73,7 @@ sub PrintClass($$@) {
   }
   print "}\n\n";
   my $n = @ranges;
-  $negname = $name;
+  my $negname = $name;
   if ($negname =~ /:/) {
     $negname =~ s/:/:^/;
   } else {
@@ -97,13 +100,25 @@ sub PrintClasses($@) {
   my $count = @entries;
 }
 
+# Prepare gofmt command
+my $gofmt;
+
+if (@ARGV > 0 && $ARGV[0] =~ /\.go$/) {
+  # Send the output of gofmt to the given file
+  open($gofmt, '|-', 'gofmt >'.$ARGV[0]) or die;
+} else {
+  open($gofmt, '|-', 'gofmt') or die;
+}
+
+# Redirect STDOUT to gofmt input
+select $gofmt;
+
 print <<EOF;
 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// GENERATED BY make_perl_groups.pl; DO NOT EDIT.
-// make_perl_groups.pl >perl_groups.go
+// Code generated by make_perl_groups.pl; DO NOT EDIT.
 
 package syntax
 

src/regexp/syntax/parse.go

@@ -1576,6 +1576,8 @@ type charGroup struct {
 	class []rune
 }
 
+//go:generate perl make_perl_groups.pl perl_groups.go
+
 // parsePerlClassEscape parses a leading Perl character class escape like \d
 // from the beginning of s. If one is present, it appends the characters to r
 // and returns the new slice r and the remainder of the string.

src/regexp/syntax/perl_groups.go

@@ -2,8 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// GENERATED BY make_perl_groups.pl; DO NOT EDIT.
-// make_perl_groups.pl >perl_groups.go
+// Code generated by make_perl_groups.pl; DO NOT EDIT.
 
 package syntax