Allow results to contain underscores

When results parsing was implemented, the set of allowed characters
wasn't made to match the rest of our substitutions. This duplicates the
set of characters though hopefully we can eventually consolidate this
logic.

I discovered this when trying to use the kaniko task in the catalog
https://github.com/tektoncd/catalog/blob/v1beta1/kaniko/kaniko.yaml

Part of tektoncd#2464.

(cherry picked from commit fc2bf79)

pkg/apis/pipeline/v1beta1/resultref.go

@@ -30,8 +30,9 @@ 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)
@@ -53,7 +54,7 @@ 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

pkg/apis/pipeline/v1beta1/resultref_test.go

@@ -203,6 +203,23 @@ 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{