=== "Python 3.10+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py39.py!}
    ```

=== "Python 3.8+"

👆 💪 🔬 <abbr title="A regular expression, regex or regexp is a sequence of characters that define a search pattern for strings.">🥔 🧬</abbr> 👈 🔢 🔜 🏏:

=== "🐍 3️⃣.6️⃣ &amp; 🔛"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004.py!}
    ```

=== "🐍 3️⃣.1️⃣0️⃣ &amp; 🔛"

    ```Python hl_lines="9"

=== "Python 3.10+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py39.py!}
    ```

=== "Python 3.8+"

=== "Python 3.10+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="11"
    {!> ../../../docs_src/query_params_str_validations/tutorial004_an_py39.py!}
    ```

=== "Python 3.8+"

```Python hl_lines="10"
{!../../../docs_src/query_params_str_validations/tutorial003.py!}
```

## 添加正则表达式

你可以定义一个参数值必须匹配的<abbr title="正则表达式或正则是定义字符串搜索模式的字符序列。">正则表达式</abbr>：

```Python hl_lines="11"
{!../../../docs_src/query_params_str_validations/tutorial004.py!}
```

这个指定的正则表达式通过以下规则检查接收到的参数值：

* `^`：以该符号之后的字符开头，符号之前没有字符。
* `fixedquery`: 值精确地等于 `fixedquery`。
* `$`: 到此结束，在 `fixedquery` 之后没有更多字符。