my $char = $self->_peek_char();

##</code><code>##

 > perl isJson.pl < file.json
Valid JSON.
Took 38s.

 > perl loops.pl
okTook 446s.

##</code><code>##

 > perl skip.pl
okTook 235s.

##</code><code>##

 > perl loops.pl
panicTook 402s.

 > perl isJson.pl < file.json
Invalid bare word (troo) at line 1020000, pos 8 (t)
Took 39s.

##</code><code>##

#!/usr/bin/perl -w
use strict;

END {
    warn "Took ", time()-$^T, "s.\n";
}

sub fail {
    my( $off, @msg ) = @_;
    my $line = $.;
    die @msg, " at line $line (end of file)\n"
        if  ! defined $off;
    my $pos = pos $_ || 0;
    my $char = substr( $_, $pos+$off, 1 );
    $pos++;
    die @msg, " at line $line, pos $pos ($char)\n";
}

my $v1 = @ARGV ? 1 : 0;
my %bare = qw< null 1 true 1 false 1 >;
my $num = qr{
    -?
    (?:
        0
     |  [1-9][0-9]*
    )
    (?: [.][0-9]+ )?
    (?: [eE][-+]?[0-9]+ )?
}x;
my @stack;
my $end = '1';
while( <STDIN> ) {
    /\G\s+/gc;
    while( ! /\G$/gc ) {
        my $in = $stack[-1] || '';
        if( '"' eq $end ) {
            while( /\G(?:[^\\"]+|\\(.))/gcs ) {
                ;
            }
            fail( -1, "Trailing \\ at end of file" )
                if  /\G\\$/gc;
            $end = ','
                if  /\G"/gc;
        } elsif( ':' eq $end ) {
            fail( 0, "Expected colon" )
                if  ! /\G:/gc;
            push @stack, ':';
            $end = '';
        } elsif( /\G([\]\}])/gc ) {
            my $got = $1;
            my $want = $end !~ /[,2]/ || ':' eq $in
                ? 'value' : $in || 'end of file';
            fail( -1, "Found close bracket, $got, when expecting $want" )
                if  $got ne $want;
            pop @stack;
            $end = ',';
        } elsif( ',' eq $end ) {
            fail( 0, "Expected end of file" )
                if  ! $in;
            fail( 0, "Expected comma" )
                if  ! /\G,/gc;
            $end = '';
        } elsif( "\}" eq $in ) {
            fail( 0, "Expected string (key)" )
                if  ! /\G"/gc;
            $end = '"';
        } elsif( /\G([\[\{])/gc ) {
            push @stack, {qw< [ ] { } >}->{$1};
            $end = '2';
        } elsif( '1' eq $end && $v1 ) {
            fail( 0, "JSON of just a scalar not allowed" );
        } elsif( /\G"/gc ) {
            $end = '"';
        } elsif( /\G$num/gc ) {
            $end = ',';
        } elsif( /\G([a-z]+)/gc ) {
            fail( -length($1), "Invalid bare word ($1)" )
                if  ! $bare{$1};
            $end = ',';
        } else {
            fail( 0, "Expected value" );
        }
        if( ',' eq $end && @stack ) {
            $end = ":"
                if  "\}" eq $stack[-1];
            pop @stack
                if  ':' eq $stack[-1];
        }
        /\G\s+/gc;
    }
}
fail( undef, "Empty JSON string" )
    if  1 eq $end;
fail( undef, "Unclosed string" )
    if  '"' eq $end;
fail( undef, "Unclosed aggregate $stack[-1]" )
    if  @stack;
warn "Valid JSON.\n";