diff --git a/duration.go b/duration.go
index 832ce5d..b690b50 100644
--- a/duration.go
+++ b/duration.go
@@ -8,7 +8,6 @@ import (
 	"strconv"
 	"strings"
 	"time"
-	"unicode"
 )
 
 // Duration holds all the smaller units that make up the duration
@@ -51,33 +50,36 @@ var (
 // Parse attempts to parse the given duration string into a *Duration,
 // if parsing fails an error is returned instead.
 func Parse(d string) (*Duration, error) {
-	state := parsingPeriod
-	duration := &Duration{}
-	num := ""
-	var err error
+	var (
+		state    = parsingPeriod
+		duration = &Duration{}
+		num      string
+		err      error
+		rank     = 8 // designator order, strictly descending: Y=7 M=6 W=5 D=4 H=3 M=2 S=1
+	)
 
 	switch {
 	case strings.HasPrefix(d, "P"): // standard duration
+		d = d[1:]
 	case strings.HasPrefix(d, "-P"): // negative duration
 		duration.Negative = true
-		d = strings.TrimPrefix(d, "-") // remove the negative sign
+		d = d[2:]
 	default:
 		return nil, ErrUnexpectedInput
 	}
 
 	for _, char := range d {
 		switch char {
-		case 'P':
-			if state != parsingPeriod {
+		case 'T':
+			if state == parsingTime || num != "" {
 				return nil, ErrUnexpectedInput
 			}
-		case 'T':
 			state = parsingTime
 		case 'Y':
-			if state != parsingPeriod {
+			if state != parsingPeriod || rank <= 7 {
 				return nil, ErrUnexpectedInput
 			}
-
+			rank = 7
 			duration.Years, err = strconv.ParseFloat(num, 64)
 			if err != nil {
 				return nil, err
@@ -85,12 +87,20 @@ func Parse(d string) (*Duration, error) {
 			num = ""
 		case 'M':
 			if state == parsingPeriod {
+				if rank <= 6 {
+					return nil, ErrUnexpectedInput
+				}
+				rank = 6
 				duration.Months, err = strconv.ParseFloat(num, 64)
 				if err != nil {
 					return nil, err
 				}
 				num = ""
-			} else if state == parsingTime {
+			} else {
+				if rank <= 2 {
+					return nil, ErrUnexpectedInput
+				}
+				rank = 2
 				duration.Minutes, err = strconv.ParseFloat(num, 64)
 				if err != nil {
 					return nil, err
@@ -98,57 +108,62 @@ func Parse(d string) (*Duration, error) {
 				num = ""
 			}
 		case 'W':
-			if state != parsingPeriod {
+			if state != parsingPeriod || rank <= 5 {
 				return nil, ErrUnexpectedInput
 			}
-
+			rank = 5
 			duration.Weeks, err = strconv.ParseFloat(num, 64)
 			if err != nil {
 				return nil, err
 			}
 			num = ""
 		case 'D':
-			if state != parsingPeriod {
+			if state != parsingPeriod || rank <= 4 {
 				return nil, ErrUnexpectedInput
 			}
-
+			rank = 4
 			duration.Days, err = strconv.ParseFloat(num, 64)
 			if err != nil {
 				return nil, err
 			}
 			num = ""
 		case 'H':
-			if state != parsingTime {
+			if state != parsingTime || rank <= 3 {
 				return nil, ErrUnexpectedInput
 			}
-
+			rank = 3
 			duration.Hours, err = strconv.ParseFloat(num, 64)
 			if err != nil {
 				return nil, err
 			}
 			num = ""
 		case 'S':
-			if state != parsingTime {
+			if state != parsingTime || rank <= 1 {
 				return nil, ErrUnexpectedInput
 			}
-
+			rank = 1
 			duration.Seconds, err = strconv.ParseFloat(num, 64)
 			if err != nil {
 				return nil, err
 			}
 			num = ""
 		default:
-			if unicode.IsNumber(char) || char == '.' {
+			if (char >= '0' && char <= '9') || char == '.' {
 				num += string(char)
 				continue
 			}
-
 			return nil, ErrUnexpectedInput
 		}
 	}
 	if num != "" {
 		return nil, ErrIncompleteExpr
 	}
+	if state == parsingPeriod && rank == 8 {
+		return nil, ErrIncompleteExpr
+	}
+	if state == parsingTime && rank > 3 {
+		return nil, ErrIncompleteExpr
+	}
 
 	return duration, nil
 }
diff --git a/duration_test.go b/duration_test.go
index 20e29e0..4cd642a 100644
--- a/duration_test.go
+++ b/duration_test.go
@@ -83,6 +83,12 @@ func TestParse(t *testing.T) {
 			},
 			errorMatchFn: noError,
 		},
+		{
+			name:         "bare P with no components",
+			args:         args{d: "P"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrIncompleteExpr),
+		},
 		{
 			name:         "no unit after prefix P",
 			args:         args{d: "P6"},
@@ -95,6 +101,84 @@ func TestParse(t *testing.T) {
 			want:         nil,
 			errorMatchFn: newMatchFn(ErrIncompleteExpr),
 		},
+		{
+			name:         "double P at start",
+			args:         args{d: "PP1D"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "trailing P in period section",
+			args:         args{d: "P1DP"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "double T",
+			args:         args{d: "PTT1H"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "T between time designators",
+			args:         args{d: "PT1HT1M"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "duplicate H designator",
+			args:         args{d: "PT1H2H"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "duplicate Y designator",
+			args:         args{d: "P1Y2Y"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "duplicate M designator in period",
+			args:         args{d: "P1M2M"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "duplicate M designator in time",
+			args:         args{d: "PT1M2M"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "time designators out of order M before H",
+			args:         args{d: "PT1M1H"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "period designators out of order D before Y",
+			args:         args{d: "P1D1Y"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "bare PT with no time components",
+			args:         args{d: "PT"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrIncompleteExpr),
+		},
+		{
+			name:         "T at end after valid time component",
+			args:         args{d: "PT2HT"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
+		{
+			name:         "non-ASCII digit",
+			args:         args{d: "P٥Y"},
+			want:         nil,
+			errorMatchFn: newMatchFn(ErrUnexpectedInput),
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {