my $FH;
my $Buf;

# Consume matching text, if found next:
sub eat {
    my( $re,            # Regex or string to match against next part of doc.
        $keep_ws,       # Don't skip over whitespace first?
        $sv,            # Append matched text to referenced scalar (if any).
        $to_end,        # Allow match to hit end of $Buf?
        $peek,          # Keep pos() unchanged?
    ) = @_;
    $Buf =~ /\G\s+/gc
        if  ! $keep_ws;
    my $start = pos( $Buf );
    $re = _compile( $re );  # Prepends \G also
    do {
        return 0
            if  $Buf !~ /$re/gc;
    } while( ! $to_end && _hit_end(\$start) );
    $$sv .= substr( $Buf, $start, pos($Buf)-$start )
        if  $sv;
    pos($Buf) = $start
        if  $peek;
    return 1;
}

# Tell me if next text matches (without consuming, except maybe whitespace):
sub peek {
    my( $re,            # Regex or string to match against next part of doc.
        $keep_ws,       # Don't skip over whitespace first.
        $sv,            # Append matched text to referenced scalar (if any).
        $to_end,        # Allow match to hit end of $Buf.
    ) = @_;
    return eat( $re, $keep_ws, $sv, $to_end, 1 );
}

# Consume matching text! If not found next, then die:
sub swallow {
    my( $re,            # Regex or string to match against next part of doc.
        $keep_ws,       # Don't skip over whitespace first.
        $sv,            # Append matched text to referenced scalar (if any).
        $to_end,        # Allow match to hit end of $Buf.
        $err,           # Error name if no match.
        @args,          # Extra details for above error.
    ) = @_;
    return 1
        if  eat( $re, $keep_ws, $sv, $to_end );
    throw( $err ||= 'Internal bug in parser', @args );
}

sub _compile {
    my( $re ) = @_;
    our %Compiled;
    return $Compiled{$re} ||= qr/\G$re/;
}

# Keep sliding window of document in $Buf:
sub _hit_end {
    my( $sv_start ) = @_;
    my $pos = pos( $Buf );
    return 0
        if  $pos < length($Buf)-1024;
    # Darn! Match got too close to end of $Buf.
    if( 2048 < $$sv_start ) {
        # Copy to left as little of $Buf as reasonable:
        my $skip = $$sv_start - 1024;
        $$sv_start -= $skip;
        # This way may just keep consuming memory:
        #   substr( $Buf, 0, $skip, '' );
        $Buf = substr( $Buf, $skip );
    }
    sysread( $FH, $Buf, 1024*1024, length($Buf) );
    # Tell caller to re-do match; have it compare at same part of $Buf again:
    pos($Buf) = $$sv_start;
    return 1;
}