如果0在字符串的开头、结尾或存在于与另一个字符分开的字符串中间，则将0加到一个数字前

$str = '5 this is 3';
$replaced = preg_replace('~(?<='s|^)'d(?='D|$)~', '0''0', $str); // 05 this is 03

正则表达式的意思是:每个数字('d)前面有一个空格或字符串的开头(?<='s|^)，后面有一个非数字或字符串的结尾(?='D|$) -替换为自己加上0

实时演示:http://ideone.com/3B7W0n

使用以下模式'/((?<= |^)[0-9](?![0-9]))/'和preg_replace():

我写了一个小测试脚本:

$pattern = '/((?<= |^)[0-9](?![0-9]))/';
$replacement = "0$1";
$tests = array(
    '5 this is 3' => '05 this is 03',
    '44 this is 2' => '44 this is 02',
    'this 4 is' => 'this 04 is',
    '44 this is' => '44 this is',
    'this is5' => 'this is5'
);
foreach($tests as $orginal => $expected) {
    $result = preg_replace($pattern, $replacement, $orginal);
    if($result !== $expected) {
        $msg  = 'Test Failed: "' . $orginal . '"' . PHP_EOL;
        $msg .= 'Expected: "' . $expected . '"' . PHP_EOL;
        $msg .= 'Got     : "' . $result . '"'. PHP_EOL;
        echo 'error' . $msg;
    } else {
        $original . '=>' . $result . PHP_EOL;
    }      
}

解释:

我使用断言确保只有数字[0-9]是:

• 后面没有数字:(?![0-9])
• ，并以空格或行首作为前置:((?<= |^)

将以0作为前缀

这是一种非正则表达式的实现方式:

$line = "this 4 is";
$words = explode(' ', $line);
foreach ($words as &$word) {
    if (ctype_digit($word)) {
        $word = str_pad($word, 2, '0', STR_PAD_LEFT);
    }
}
echo implode(' ', $words);