Allow escaping $ in default values (#128)

Author: sethvargo

README.md

@@ -93,6 +93,24 @@ examples.
     }
     ```
 
+    As a special case where the default value should contain a literal `$`,
+    escape it with a backslash. Unfortunately this requires a double backslash
+    in the struct tag:
+
+    ```go
+    type MyStruct struct {
+      Amount string `env:"AMOUNT, default=\\$5.00"` // Default: $5.00
+    }
+    ```
+
+    To have a literal backslash followed by a `$`, escape the backslash:
+
+    ```go
+    type MyStruct struct {
+      Filepath string `env:"FILEPATH, default=C:\\Personal\\\\$name"` // Default: C:\Personal\$name
+    }
+    ```
+
 -   `prefix` - sets the prefix to use for looking up environment variable keys
     on child structs and fields. This is useful for shared configurations:
 

envconfig.go

@@ -642,6 +642,18 @@ func lookup(key string, required bool, defaultValue string, l Lookuper) (string,
 		}
 
 		if defaultValue != "" {
+			// Handle escaped "$" by replacing the value with a character that is
+			// invalid to have in an environment variable. A more perfect solution
+			// would be to re-implement os.Expand to handle this case, but that's been
+			// proposed and rejected in the stdlib. Additionally, the function is
+			// dependent on other private functions in the [os] package, so
+			// duplicating it is toilsome.
+			//
+			// While admittidly a hack, replacing the escaped values with invalid
+			// characters (and then replacing later), is a reasonable solution.
+			defaultValue = strings.ReplaceAll(defaultValue, "\\\\", "\u0000")
+			defaultValue = strings.ReplaceAll(defaultValue, "\\$", "\u0008")
+
 			// Expand the default value. This allows for a default value that maps to
 			// a different environment variable.
 			val = os.Expand(defaultValue, func(i string) string {
@@ -657,6 +669,9 @@ func lookup(key string, required bool, defaultValue string, l Lookuper) (string,
 				return ""
 			})
 
+			val = strings.ReplaceAll(val, "\u0000", "\\")
+			val = strings.ReplaceAll(val, "\u0008", "$")
+
 			return val, false, true, nil
 		}
 	}

envconfig_test.go

@@ -1350,6 +1350,34 @@ func TestProcessWith(t *testing.T) {
 				"DEFAULT": "value",
 			}))),
 		},
+		{
+			name: "default/escaped_doesnt_interpolate",
+			target: &struct {
+				Field string `env:"FIELD,default=\\$DEFAULT"`
+			}{},
+			exp: &struct {
+				Field string `env:"FIELD,default=\\$DEFAULT"`
+			}{
+				Field: "$DEFAULT",
+			},
+			lookuper: MapLookuper(map[string]string{
+				"DEFAULT": "should-not-be-replaced",
+			}),
+		},
+		{
+			name: "default/escaped_escaped_keeps_escape",
+			target: &struct {
+				Field string `env:"FIELD,default=C:\\Personal\\\\$DEFAULT"`
+			}{},
+			exp: &struct {
+				Field string `env:"FIELD,default=C:\\Personal\\\\$DEFAULT"`
+			}{
+				Field: `C:\Personal\value`,
+			},
+			lookuper: MapLookuper(map[string]string{
+				"DEFAULT": "value",
+			}),
+		},
 		{
 			name: "default/slice",
 			target: &struct {