diff --git a/pkg/apis/pipeline/v1beta1/resultref.go b/pkg/apis/pipeline/v1beta1/resultref.go
index b4f16998a28..478cb62c479 100644
--- a/pkg/apis/pipeline/v1beta1/resultref.go
+++ b/pkg/apis/pipeline/v1beta1/resultref.go
@@ -33,8 +33,9 @@ const (
 	// ResultTaskPart Constant used to define the "tasks" part of a pipeline result reference
 	ResultTaskPart = "tasks"
 	// ResultResultPart Constant used to define the "results" part of a pipeline result reference
-	ResultResultPart           = "results"
-	variableSubstitutionFormat = `\$\([A-Za-z0-9-]+(\.[A-Za-z0-9-]+)*\)`
+	ResultResultPart = "results"
+	// TODO(#2462) use one regex across all substitutions
+	variableSubstitutionFormat = `\$\([_a-zA-Z0-9.-]+(\.[_a-zA-Z0-9.-]+)*\)`
 )
 
 var variableSubstitutionRegex = regexp.MustCompile(variableSubstitutionFormat)
@@ -56,7 +57,7 @@ func NewResultRefs(expressions []string) ([]*ResultRef, error) {
 	return resultRefs, nil
 }
 
-// looksLikeContainsResultRefs attempts to check if param or a pipeline result looks like it contains any
+// LooksLikeContainsResultRefs attempts to check if param or a pipeline result looks like it contains any
 // result references.
 // This is useful if we want to make sure the param looks like a ResultReference before
 // performing strict validation
diff --git a/pkg/apis/pipeline/v1beta1/resultref_test.go b/pkg/apis/pipeline/v1beta1/resultref_test.go
index d2b55a1ad23..de488ff75e3 100644
--- a/pkg/apis/pipeline/v1beta1/resultref_test.go
+++ b/pkg/apis/pipeline/v1beta1/resultref_test.go
@@ -206,6 +206,23 @@ func TestHasResultReference(t *testing.T) {
 					Result:       "sum-result",
 				},
 			},
+		}, {
+			name: "Test valid expression with underscores",
+			args: args{
+				param: v1beta1.Param{
+					Name: "param",
+					Value: v1beta1.ArrayOrString{
+						Type:      v1beta1.ParamTypeString,
+						StringVal: "$(tasks.sum-task.results.sum_result)",
+					},
+				},
+			},
+			wantRef: []*v1beta1.ResultRef{
+				{
+					PipelineTask: "sum-task",
+					Result:       "sum_result",
+				},
+			},
 		}, {
 			name: "Test invalid expression: param substitution shouldn't be considered result ref",
 			args: args{