docs: document default env placeholder

docs/config.md

@@ -174,7 +174,7 @@ go-cqhttp 配置文件可以使用占位符来读取**环境变量**的值。
 ```yaml
 account: # 账号相关
   uin: ${CQ_UIN} # 读取环境变量 CQ_UIN
-  password: ${CQ_PASSWORD} # 读取环境变量 CQ_UIN
+  password: ${CQ_PWD:123456} # 当 CQ_PWD 为空时使用默认值 123456
 ```
 
 ## 在线状态

modules/config/config.go

@@ -152,14 +152,14 @@ func generateConfig() {
 // os.ExpandEnv 字符 $ 无法逃逸
 // https://github.com/golang/go/issues/43482
 func expand(s string, mapping func(string) string) string {
-	r := regexp.MustCompile(`\${([a-zA-Z_]+[a-zA-Z0-9_:]*)}`)
+	r := regexp.MustCompile(`\${([a-zA-Z_]+[a-zA-Z0-9_:/.]*)}`)
 	return r.ReplaceAllStringFunc(s, func(s string) string {
 		s = strings.Trim(s, "${}")
 		// todo: use strings.Cut once go1.18 is released
-		placeholder, default_, ok := cut(s, ":")
-		m := mapping(placeholder)
+		before, after, ok := cut(s, ":")
+		m := mapping(before)
 		if ok && m == "" {
-			return default_
+			return after
 		}
 		return m
 	})

modules/config/config_test.go

@@ -6,6 +6,9 @@ import (
 )
 
 func Test_expand(t *testing.T) {
+	nullStringMapping := func(_ string) string {
+		return ""
+	}
 	tests := []struct {
 		src      string
 		mapping  func(string) string
@@ -23,9 +26,19 @@ func Test_expand(t *testing.T) {
 		},
 		{
 			src:      "foo: ${bar:123456}",
-			mapping:  func(s string) string { return "" },
+			mapping:  nullStringMapping,
 			expected: "foo: 123456",
 		},
+		{
+			src:      "foo: ${bar:127.0.0.1:5700}",
+			mapping:  nullStringMapping,
+			expected: "foo: 127.0.0.1:5700",
+		},
+		{
+			src:      "foo: ${bar:ws//localhost:9999/ws}",
+			mapping:  nullStringMapping,
+			expected: "foo: ws//localhost:9999/ws",
+		},
 	}
 	for i, tt := range tests {
 		if got := expand(tt.src, tt.mapping); got != tt.expected {