diff --git a/size.go b/size.go
index 85f6ab0..c245a89 100644
--- a/size.go
+++ b/size.go
@@ -2,7 +2,6 @@ package units
 
 import (
 	"fmt"
-	"regexp"
 	"strconv"
 	"strings"
 )
@@ -26,16 +25,17 @@ const (
 	PiB = 1024 * TiB
 )
 
-type unitMap map[string]int64
+type unitMap map[byte]int64
 
 var (
-	decimalMap = unitMap{"k": KB, "m": MB, "g": GB, "t": TB, "p": PB}
-	binaryMap  = unitMap{"k": KiB, "m": MiB, "g": GiB, "t": TiB, "p": PiB}
-	sizeRegex  = regexp.MustCompile(`^(\d+(\.\d+)*) ?([kKmMgGtTpP])?[iI]?[bB]?$`)
+	decimalMap = unitMap{'k': KB, 'm': MB, 'g': GB, 't': TB, 'p': PB}
+	binaryMap  = unitMap{'k': KiB, 'm': MiB, 'g': GiB, 't': TiB, 'p': PiB}
 )
 
-var decimapAbbrs = []string{"B", "kB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"}
-var binaryAbbrs = []string{"B", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"}
+var (
+	decimapAbbrs = []string{"B", "kB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB"}
+	binaryAbbrs  = []string{"B", "KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"}
+)
 
 func getSizeAndUnit(size float64, base float64, _map []string) (float64, string) {
 	i := 0
@@ -89,20 +89,66 @@ func RAMInBytes(size string) (int64, error) {
 
 // Parses the human-readable size string into the amount it represents.
 func parseSize(sizeStr string, uMap unitMap) (int64, error) {
-	matches := sizeRegex.FindStringSubmatch(sizeStr)
-	if len(matches) != 4 {
+	// TODO: rewrite to use strings.Cut if there's a space
+	// once Go < 1.18 is deprecated.
+	sep := strings.LastIndexAny(sizeStr, "01234567890. ")
+	if sep == -1 {
+		// There should be at least a digit.
 		return -1, fmt.Errorf("invalid size: '%s'", sizeStr)
 	}
+	var num, sfx string
+	if sizeStr[sep] != ' ' {
+		num = sizeStr[:sep+1]
+		sfx = sizeStr[sep+1:]
+	} else {
+		// Omit the space separator.
+		num = sizeStr[:sep]
+		sfx = sizeStr[sep+1:]
+	}
 
-	size, err := strconv.ParseFloat(matches[1], 64)
+	size, err := strconv.ParseFloat(num, 64)
 	if err != nil {
 		return -1, err
 	}
+	// Backward compatibility: reject negative sizes.
+	if size < 0 {
+		return -1, fmt.Errorf("invalid size: '%s'", sizeStr)
+	}
+
+	if len(sfx) == 0 {
+		return int64(size), nil
+	}
 
-	unitPrefix := strings.ToLower(matches[3])
-	if mul, ok := uMap[unitPrefix]; ok {
+	// Process the suffix.
+
+	if len(sfx) > 3 { // Too long.
+		goto badSuffix
+	}
+	sfx = strings.ToLower(sfx)
+	// Trivial case: b suffix.
+	if sfx[0] == 'b' {
+		if len(sfx) > 1 { // no extra characters allowed after b.
+			goto badSuffix
+		}
+		return int64(size), nil
+	}
+	// A suffix from the map.
+	if mul, ok := uMap[sfx[0]]; ok {
 		size *= float64(mul)
+	} else {
+		goto badSuffix
+	}
+
+	// The suffix may have extra "b" or "ib" (e.g. KiB or MB).
+	switch {
+	case len(sfx) == 2 && sfx[1] != 'b':
+		goto badSuffix
+	case len(sfx) == 3 && sfx[1:] != "ib":
+		goto badSuffix
 	}
 
 	return int64(size), nil
+
+badSuffix:
+	return -1, fmt.Errorf("invalid suffix: '%s'", sfx)
 }
diff --git a/size_test.go b/size_test.go
index 3ab06a1..06b3cd6 100644
--- a/size_test.go
+++ b/size_test.go
@@ -103,6 +103,22 @@ func TestFromHumanSize(t *testing.T) {
 	assertSuccessEquals(t, 32.5*KB, FromHumanSize, "32.5 kB")
 	assertSuccessEquals(t, 32, FromHumanSize, "32.5 B")
 	assertSuccessEquals(t, 300, FromHumanSize, "0.3 K")
+	assertSuccessEquals(t, 300, FromHumanSize, ".3kB")
+
+	assertSuccessEquals(t, 0, FromHumanSize, "0.")
+	assertSuccessEquals(t, 0, FromHumanSize, "0. ")
+	assertSuccessEquals(t, 0, FromHumanSize, "0.b")
+	assertSuccessEquals(t, 0, FromHumanSize, "0.B")
+	assertSuccessEquals(t, 0, FromHumanSize, "-0")
+	assertSuccessEquals(t, 0, FromHumanSize, "-0b")
+	assertSuccessEquals(t, 0, FromHumanSize, "-0B")
+	assertSuccessEquals(t, 0, FromHumanSize, "-0 b")
+	assertSuccessEquals(t, 0, FromHumanSize, "-0 B")
+	assertSuccessEquals(t, 32, FromHumanSize, "32.")
+	assertSuccessEquals(t, 32, FromHumanSize, "32.b")
+	assertSuccessEquals(t, 32, FromHumanSize, "32.B")
+	assertSuccessEquals(t, 32, FromHumanSize, "32. b")
+	assertSuccessEquals(t, 32, FromHumanSize, "32. B")
 
 	// We do not tolerate extra leading or trailing spaces
 	// (except for a space after the number and a missing suffix).
@@ -124,26 +140,11 @@ func TestFromHumanSize(t *testing.T) {
 	assertError(t, FromHumanSize, "  ")
 	assertError(t, FromHumanSize, " .")
 	assertError(t, FromHumanSize, " . ")
-	assertError(t, FromHumanSize, "0.")
-	assertError(t, FromHumanSize, "0. ")
-	assertError(t, FromHumanSize, "0.b")
-	assertError(t, FromHumanSize, "0.B")
-	assertError(t, FromHumanSize, "-0")
-	assertError(t, FromHumanSize, "-0b")
-	assertError(t, FromHumanSize, "-0B")
-	assertError(t, FromHumanSize, "-0 b")
-	assertError(t, FromHumanSize, "-0 B")
 	assertError(t, FromHumanSize, "-32")
-	assertError(t, FromHumanSize, ".3kB")
 	assertError(t, FromHumanSize, "-32b")
 	assertError(t, FromHumanSize, "-32B")
 	assertError(t, FromHumanSize, "-32 b")
 	assertError(t, FromHumanSize, "-32 B")
-	assertError(t, FromHumanSize, "32.")
-	assertError(t, FromHumanSize, "32.b")
-	assertError(t, FromHumanSize, "32.B")
-	assertError(t, FromHumanSize, "32. b")
-	assertError(t, FromHumanSize, "32. B")
 	assertError(t, FromHumanSize, "32b.")
 	assertError(t, FromHumanSize, "32B.")
 	assertError(t, FromHumanSize, "32 b.")