使用pattern屬性進行表單驗證的關鍵在於正確編寫正則表達式，並註意其適用範圍與限制。 1. 在HTML的input元素中添加pattern屬性並設置正則表達式，可實現(xiàn)無需JavaScript的輸入格式校驗；2. 常見用例包括手機號、密碼、用戶名等格式驗證；3. 默認錯誤信息可通過JavaScript自定義以提升用戶體驗；4. 注意pattern僅在提交時驗證、並非所有input類型支持、且需配合服務端驗證。合理利用pattern能簡化前端驗證邏輯，但必須理解其工作機制和局限性。

Using the pattern attribute for custom form validation is a simple and effective way to ensure that user input matches a specific format — all without writing any JavaScript. It works directly in HTML, mainly with <input> elements, and uses regular expressions (regex) to define what kind of input is acceptable.

Here's how you can use it effectively:

Basic Usage: Apply Pattern Directly in HTML

To use the pattern attribute, simply add it to your input element and set its value to a regex pattern that defines what valid input looks like.

For example, if you want an input field where users must enter only letters (both uppercase and lowercase), you could write:

 <input type="text" pattern="[A-Za-z] " required>

This means:

• [A-Za-z] allows only letters.
• The means "one or more" of those characters.
• The required attribute ensures the field can't be left blank.

If someone tries to submit the form with anything other than letters, the browser will show a default error message.

Common Patterns for Different Use Cases

You don't need to reinvent the wheel. Here are some commonly used patterns for different validation needs:

• Phone number (10 digits):

   <input type="text" pattern="\d{10}" required>

  This requires exactly 10 digits, no spaces or dashes.

• Password with at least one letter and one number:

   <input type="password" pattern="(?=.*\d)(?=.*[a-zA-Z]).{8,}" required>

  This checks for at least one digit, one letter, and a minimum length of 8 characters.

• Username with letters, numbers, underscores, and hyphens:

   <input type="text" pattern="[\w\-] " required>

These examples can be tweaked based on your exact requirements. Just remember that browsers interpret these patterns using JavaScript-style regular expressions.

Handling Error Messages

By default, browsers show generic error messages when the input doesn't match the pattern. If you want to customize that message, you'll need a bit of JavaScript:

 <input type="text" id="username" pattern="[A-Za-z] " required>
<small>Only letters allowed</small>

<script>
  const input = document.getElementById(&#39;username&#39;);
  input.addEventListener(&#39;invalid&#39;, function(e) {
    e.preventDefault(); // prevent default message
    if (input.validity.patternMismatch) {
      input.setCustomValidity(&#39;Please enter only alphabetical characters.&#39;);
    }
  });
</script>

This lets you give users clearer feedback while still relying mostly on HTML for validation logic.

One thing to note: setCustomValidity() resets after every successful input change, so you don't have to worry about manually clearing it unless you're doing something advanced.

Things to Keep in Mind

• Pattern only validates on form submission , not as the user types.
• Not all input types support pattern . It works best with type="text" , type="password" , etc.
• Always validate again on the server side — client-side validation is just for convenience.

So, basically, the pattern attribute gives you powerful validation tools right inside your HTML, as long as you know how to write the regex and understand its limitations.

基本上就這些。

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