diff --git a/filters/ref-confidence.go b/filters/ref-confidence.go
index ca581c1..817dee7 100644
--- a/filters/ref-confidence.go
+++ b/filters/ref-confidence.go
@@ -45,14 +45,14 @@ func getBasesAndBaseQualitiesAlignedOneToOne(aln *sam.Alignment) (sam.Sequence,
 			paddedBaseQualities := make([]byte, 0, nofRefBases)
 			var pos int32
 			for _, element := range aln.CIGAR {
-				if operatorConsumesReadBases[element.Operation] {
+				if operatorConsumesReadBases.Contains(element.Operation) {
 					end := pos + element.Length
-					if operatorConsumesReferenceBases[element.Operation] {
+					if operatorConsumesReferenceBases.Contains(element.Operation) {
 						paddedBases = paddedBases.AppendSlice(nibbles.Nibbles(bases.Slice(int(pos), int(end))))
 						paddedBaseQualities = append(paddedBaseQualities, baseQualities[pos:end]...)
 					}
 					pos = end
-				} else if operatorConsumesReferenceBases[element.Operation] {
+				} else if operatorConsumesReferenceBases.Contains(element.Operation) {
 					for j := int32(0); j < element.Length; j++ {
 						paddedBases = paddedBases.Append('-')
 						paddedBaseQualities = append(paddedBaseQualities, 0)
diff --git a/filters/simple-filters.go b/filters/simple-filters.go
index 7d7e56a..b8e0d7a 100644
--- a/filters/simple-filters.go
+++ b/filters/simple-filters.go
@@ -23,6 +23,7 @@ import (
 	"math/rand"
 	"strconv"
 
+	"github.com/elliotwutingfeng/asciiset"
 	"github.com/exascience/elprep/v5/bed"
 	"github.com/exascience/elprep/v5/intervals"
 	"github.com/exascience/elprep/v5/sam"
@@ -82,7 +83,7 @@ func RemoveUnmappedReadsStrict(_ *sam.Header) sam.AlignmentFilter {
 	}
 }
 
-var nonExactMappingOperator = map[byte]bool{'I': true, 'D': true, 'N': true, 'H': true, 'P': true, 'X': true, '=': true}
+var nonExactMappingOperator, _ = asciiset.MakeASCIISet("IDNHPX=")
 
 // RemoveNonExactMappingReads is a filter that removes all reads that
 // are not exact matches with the reference (soft-clipping ok), based
@@ -90,7 +91,7 @@ var nonExactMappingOperator = map[byte]bool{'I': true, 'D': true, 'N': true, 'H'
 func RemoveNonExactMappingReads(_ *sam.Header) sam.AlignmentFilter {
 	return func(aln *sam.Alignment) bool {
 		for _, op := range aln.CIGAR {
-			if nonExactMappingOperator[op.Operation] {
+			if nonExactMappingOperator.Contains(op.Operation) {
 				return false
 			}
 		}
diff --git a/filters/utils.go b/filters/utils.go
index 99d8dc5..f18b0cc 100644
--- a/filters/utils.go
+++ b/filters/utils.go
@@ -21,6 +21,7 @@ package filters
 import (
 	"log"
 
+	"github.com/elliotwutingfeng/asciiset"
 	"github.com/exascience/elprep/v5/sam"
 )
 
@@ -74,13 +75,13 @@ func absInt32(x int32) int32 {
 }
 
 var (
-	operatorConsumesReadBases      = map[byte]bool{'M': true, 'I': true, 'S': true, '=': true, 'X': true}
-	operatorConsumesReferenceBases = map[byte]bool{'M': true, 'D': true, 'N': true, '=': true, 'X': true}
+	operatorConsumesReadBases, _      = asciiset.MakeASCIISet("MIS=X")
+	operatorConsumesReferenceBases, _ = asciiset.MakeASCIISet("MDN=X")
 )
 
 func elementStradlessClippedRead(newCigar []sam.CigarOperation, operator byte, relativeClippingPosition, clippedBases int32) []sam.CigarOperation {
-	if operatorConsumesReadBases[operator] {
-		if operatorConsumesReferenceBases[operator] {
+	if operatorConsumesReadBases.Contains(operator) {
+		if operatorConsumesReferenceBases.Contains(operator) {
 			if relativeClippingPosition > 0 {
 				newCigar = append(newCigar, sam.CigarOperation{
 					Length:    relativeClippingPosition,
@@ -279,7 +280,7 @@ func computeReadCoordinateForReferenceCoordinate(cigarVec []sam.CigarOperation,
 		index++
 		elementLength := int(element.Length)
 		var shift int
-		if operatorConsumesReferenceBases[element.Operation] || element.Operation == 'S' {
+		if operatorConsumesReferenceBases.Contains(element.Operation) || element.Operation == 'S' {
 			if refBases+elementLength < goal {
 				shift = elementLength
 			} else {
diff --git a/go.mod b/go.mod
index fd45a86..136a04f 100644
--- a/go.mod
+++ b/go.mod
@@ -4,6 +4,7 @@ go 1.18
 
 require (
 	github.com/bits-and-blooms/bitset v1.2.2
+	github.com/elliotwutingfeng/asciiset v0.0.0-20230602022725-51bbb787efab
 	github.com/exascience/pargo v1.1.0
 	github.com/google/uuid v1.3.0
 	golang.org/x/sys v0.0.0-20220422013727-9388b58f7150
diff --git a/go.sum b/go.sum
index aa56fd8..d9fcf74 100644
--- a/go.sum
+++ b/go.sum
@@ -1,6 +1,8 @@
 github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af/go.mod h1:K08gAheRH3/J6wwsYMMT4xOr94bZjxIelGM0+d/wbFw=
 github.com/bits-and-blooms/bitset v1.2.2 h1:J5gbX05GpMdBjCvQ9MteIg2KKDExr7DrgK+Yc15FvIk=
 github.com/bits-and-blooms/bitset v1.2.2/go.mod h1:gIdJ4wp64HaoK2YrL1Q5/N7Y16edYb8uY+O0FJTyyDA=
+github.com/elliotwutingfeng/asciiset v0.0.0-20230602022725-51bbb787efab h1:h1UgjJdAAhj+uPL68n7XASS6bU+07ZX1WJvVS2eyoeY=
+github.com/elliotwutingfeng/asciiset v0.0.0-20230602022725-51bbb787efab/go.mod h1:GLo/8fDswSAniFG+BFIaiSPcK610jyzgEhWYPQwuQdw=
 github.com/exascience/pargo v1.1.0 h1:pBKDhJYoH2ekBehnPCErSIDoi9DqiWL1V70s5kGZANI=
 github.com/exascience/pargo v1.1.0/go.mod h1:8GeMktPA5KycHMfqXXOfiQzlazfbFSURzGZIJUO0tfk=
 github.com/fogleman/gg v1.2.1-0.20190220221249-0403632d5b90/go.mod h1:R/bRT+9gY/C5z7JzPU0zXsXHKM4/ayA+zqcVNZzPa1k=