diff --git a/ast/expr.go b/ast/expr.go
index 3af59df3..71314d86 100644
--- a/ast/expr.go
+++ b/ast/expr.go
@@ -5,15 +5,12 @@ package ast
 import (
 	"encoding/json"
 	"fmt"
-	"regexp"
 	"strings"
 
 	"github.com/hashicorp/hcl/v2"
 	"github.com/pulumi/esc/syntax"
 )
 
-var fnOpenRegex = regexp.MustCompile(`^fn::open::[a-zA-Z-]+$`)
-
 // Expr represents a Pulumi YAML expression. Expressions may be literals, interpolated strings, symbols, or builtin
 // functions.
 type Expr interface {
@@ -534,7 +531,7 @@ func tryParseFunction(node *syntax.ObjectNode) (Expr, syntax.Diagnostics, bool)
 	case "fn::toString":
 		parse = parseToString
 	default:
-		if match, _ := regexp.MatchString(fnOpenRegex.String(), kvp.Key.Value()); match {
+		if strings.HasPrefix(kvp.Key.Value(), "fn::open::") {
 			// transform the node into fn::open format
 			providerName := strings.TrimPrefix(kvp.Key.Value(), "fn::")
 			// case 1: inputs are provided