如何将mb_convert_case与正则表达式配合使用实现灵活的大小写转换？

gitbox 2025-09-19

1. mb_convert_case 的基本用法

mb_convert_case 函数可以将字符串中的字符按指定的大小写转换，支持三种模式：

MB_CASE_UPPER：转换为全大写。
MB_CASE_LOWER：转换为全小写。
MB_CASE_TITLE：将每个单词的首字母转换为大写，其余字母小写。

示例代码：

<span><span><span class="hljs-variable">$string</span></span><span> = </span><span><span class="hljs-string">"hello world! 你好，世界！"</span></span><span>;

</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$string</span></span><span>, MB_CASE_UPPER, </span><span><span class="hljs-string">"UTF-8"</span></span><span>);  </span><span><span class="hljs-comment">// 输出：HELLO WORLD! 你好，世界！</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$string</span></span><span>, MB_CASE_LOWER, </span><span><span class="hljs-string">"UTF-8"</span></span><span>);  </span><span><span class="hljs-comment">// 输出：hello world! 你好，世界！</span></span><span>
</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$string</span></span><span>, MB_CASE_TITLE, </span><span><span class="hljs-string">"UTF-8"</span></span><span>); </span><span><span class="hljs-comment">// 输出：Hello World! 你好，世界！</span></span><span>
</span></span>

2. 正则表达式的作用

正则表达式（RegEx）是一种强大的字符串模式匹配工具。在处理复杂的字符串转换时，正则表达式能够帮助我们精准地定位需要转换的部分，比如特定的单词、字母或符号，进而对这些部分进行大小写转换。

通过正则表达式，我们可以定义一些规则，结合 mb_convert_case 对匹配到的部分进行转换，达到更加灵活的效果。

3. 将 mb_convert_case 与正则表达式结合

3.1 例子：将字符串中的每个单词首字母大写，其他字母小写

假设我们有一个字符串，需要将其中的每个单词首字母大写，其他字母小写，但同时我们希望保留某些特定的单词（如“PHP”）的大小写不受影响。这个时候，可以通过正则表达式找到每个单词并使用 mb_convert_case 来实现。

示例代码：

<span><span><span class="hljs-variable">$string</span></span><span> = </span><span><span class="hljs-string">"this is a php test, check PHP again."</span></span><span>;

</span><span><span class="hljs-variable">$pattern</span></span><span> = </span><span><span class="hljs-string">'/\b(\w+)\b/'</span></span><span>; </span><span><span class="hljs-comment">// 匹配单词边界</span></span><span>
</span><span><span class="hljs-variable">$callback</span></span><span> = </span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> (</span><span><span class="hljs-params"><span class="hljs-variable">$matches</span></span></span><span>) {
    </span><span><span class="hljs-comment">// 对匹配的单词进行首字母大写处理</span></span><span>
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$matches</span></span><span>[</span><span><span class="hljs-number">0</span></span><span>], MB_CASE_TITLE, </span><span><span class="hljs-string">'UTF-8'</span></span><span>);
};

</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">preg_replace_callback</span></span><span>(</span><span><span class="hljs-variable">$pattern</span></span><span>, </span><span><span class="hljs-variable">$callback</span></span><span>, </span><span><span class="hljs-variable">$string</span></span><span>);

</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$result</span></span><span>;  </span><span><span class="hljs-comment">// 输出：This Is A Php Test, Check Php Again.</span></span><span>
</span></span>

在这个例子中，正则表达式 /\b(\w+)\b/ 匹配了所有单词边界的内容（即每个单词），然后通过 preg_replace_callback 回调函数，使用 mb_convert_case 将每个单词的首字母转换为大写，其余字母小写。

3.2 例子：对指定单词进行大小写转换

有时我们可能只需要对某些特定的单词进行大小写转换，而不影响其他内容。例如，我们可以将所有的“php”转为“PHP”，而其他单词保持不变。

示例代码：

<span><span><span class="hljs-variable">$string</span></span><span> = </span><span><span class="hljs-string">"I love php, and I am learning php in 2025."</span></span><span>;

</span><span><span class="hljs-variable">$pattern</span></span><span> = </span><span><span class="hljs-string">'/\bphp\b/i'</span></span><span>;  </span><span><span class="hljs-comment">// 匹配“php”，不区分大小写</span></span><span>
</span><span><span class="hljs-variable">$callback</span></span><span> = </span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> (</span><span><span class="hljs-params"><span class="hljs-variable">$matches</span></span></span><span>) {
    </span><span><span class="hljs-comment">// 将匹配到的“php”转换为大写</span></span><span>
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$matches</span></span><span>[</span><span><span class="hljs-number">0</span></span><span>], MB_CASE_UPPER, </span><span><span class="hljs-string">'UTF-8'</span></span><span>);
};

</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">preg_replace_callback</span></span><span>(</span><span><span class="hljs-variable">$pattern</span></span><span>, </span><span><span class="hljs-variable">$callback</span></span><span>, </span><span><span class="hljs-variable">$string</span></span><span>);

</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$result</span></span><span>;  </span><span><span class="hljs-comment">// 输出：I love PHP, and I am learning PHP in 2025.</span></span><span>
</span></span>

这里的正则表达式 /\bphp\b/i 匹配了所有不区分大小写的“php”单词。mb_convert_case 将它们转换为大写。

4. 高级应用：根据字符的上下文条件进行转换

在一些复杂的场景中，可能需要根据字符在上下文中的位置来决定是否转换大小写。比如，某些缩写或代码片段（如“API”）应保持原样，而其他单词则可以进行大小写转换。

示例代码：

<span><span><span class="hljs-variable">$string</span></span><span> = </span><span><span class="hljs-string">"the API is a powerful tool, API is awesome."</span></span><span>;

</span><span><span class="hljs-comment">// 使用负向前瞻来排除已出现的“API”</span></span><span>
</span><span><span class="hljs-comment">// 这样就可以保证“API”不被转换，而其他单词正常转换</span></span><span>
</span><span><span class="hljs-variable">$pattern</span></span><span> = </span><span><span class="hljs-string">'/\b(?!API\b)\w+\b/'</span></span><span>;
</span><span><span class="hljs-variable">$callback</span></span><span> = </span><span><span class="hljs-function"><span class="hljs-keyword">function</span></span></span><span> (</span><span><span class="hljs-params"><span class="hljs-variable">$matches</span></span></span><span>) {
    </span><span><span class="hljs-keyword">return</span></span><span> </span><span><span class="hljs-title function_ invoke__">mb_convert_case</span></span><span>(</span><span><span class="hljs-variable">$matches</span></span><span>[</span><span><span class="hljs-number">0</span></span><span>], MB_CASE_TITLE, </span><span><span class="hljs-string">'UTF-8'</span></span><span>);
};

</span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">preg_replace_callback</span></span><span>(</span><span><span class="hljs-variable">$pattern</span></span><span>, </span><span><span class="hljs-variable">$callback</span></span><span>, </span><span><span class="hljs-variable">$string</span></span><span>);

</span><span><span class="hljs-keyword">echo</span></span><span> </span><span><span class="hljs-variable">$result</span></span><span>;  </span><span><span class="hljs-comment">// 输出：The API is A Powerful Tool, API is Awesome.</span></span><span>
</span></span>

在这个例子中，我们通过负向前瞻 (?!API\b) 来确保“API”不被转换，同时其他单词都进行了首字母大写的转换。

5. 小结

结合 mb_convert_case 和正则表达式，可以实现更为复杂和灵活的大小写转换。这种方法尤其适用于需要根据特定规则（如匹配特定单词、排除某些部分或根据上下文）来控制字符大小写的场景。无论是简单的全局大小写转换，还是需要精细控制的情形，mb_convert_case 和正则表达式的组合都能提供非常高效且定制化的解决方案。