使用preg_replace转换缩进（无回调）

您可以使用 ：\G

preg_replace('/^  |\G  /m', "\t", $string);

做了一些基准测试，并在Win32上使用PHP 5.2和5.4获得了以下结果：

>php -v
PHP 5.2.17 (cli) (built: Jan  6 2011 17:28:41)
Copyright (c) 1997-2010 The PHP Group
Zend Engine v2.2.0, Copyright (c) 1998-2010 Zend Technologies

>php -n test.php
XML length: 21100
Iterations: 1000
callback: 2.3627231121063
\G:       1.4221360683441
while:    3.0971200466156
/e:       7.8781840801239


>php -v
PHP 5.4.0 (cli) (built: Feb 29 2012 19:06:50)
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2012 Zend Technologies

>php -n test.php
XML length: 21100
Iterations: 1000
callback: 1.3771259784698
\G:       1.4414191246033
while:    2.7389969825745
/e:       5.5516891479492

令人惊讶的是，回调比PHP 5.4更快（尽管似乎依赖于数据，但在其他一些情况下更快）。\G\G

For 是使用的，并且比 快一点。 甚至比 慢。、、开关没有明显影响性能。\G/^ |\G /m/(?:^|\G) /m/(?>^|\G) /m/(?:^|\G) /m/u/S/X\G

如果深度较低（在我的测试中，最多约 4 个缩进，8 个空格），则替换速度最快，但随着深度的增加，替换速度会变慢。while

使用了以下代码：

<?php

$base_iter = 1000;

$xml_string = str_repeat(<<<_STR_
<?xml version="1.0"?>
<root>
  <error>
    <a>  eee  </a>
    <b>  sd    </b>         
    <c>
            deep
                deeper  still
                    deepest  !
    </c>
  </error>
</root>
_STR_
, 100);


//*** while ***

$re = '%# Match leading spaces following leading tabs.
    ^                     # Anchor to start of line.
    (\t*)                 # $1: Preserve any/all leading tabs.
    [ ]{2}                # Match "n" spaces.
    %mx';

function conv_indent_while($xml_string) {
    global $re;

    while(preg_match($re, $xml_string))
        $xml_string = preg_replace($re, "$1\t", $xml_string);

    return $xml_string;
}


//*** \G ****

function conv_indent_g($string){
    return preg_replace('/^  |\G  /m', "\t", $string);
}


//*** callback ***

function callback($m)
{
    $spaces = strlen($m[0]);
    $tabs = $spaces / 2;
    return str_repeat("\t", $tabs);
}
function conv_indent_callback($str){
    return preg_replace_callback('/^(?:[ ]{2})+/m', 'callback', $str);
}


//*** callback /e *** 

function conv_indent_e($str){
    return preg_replace('/^(?:  )+/me', 'str_repeat("\t", strlen("$0")/2)', $str);
}



//*** tests

function test2() {
    global $base_iter;
    global $xml_string;
    $t = microtime(true);

    for($i = 0; $i < $base_iter; ++$i){
        $s = conv_indent_while($xml_string);
        if(strlen($s) >= strlen($xml_string))
            exit("strlen invalid 2");
    }

    return (microtime(true) - $t);
}

function test1() {
    global $base_iter;
    global $xml_string;
    $t = microtime(true);

    for($i = 0; $i < $base_iter; ++$i){
        $s = conv_indent_g($xml_string);
        if(strlen($s) >= strlen($xml_string))
            exit("strlen invalid 1");
    }

    return (microtime(true) - $t);
}

function test0(){
    global $base_iter;
    global $xml_string;
    $t = microtime(true);

    for($i = 0; $i < $base_iter; ++$i){     
        $s = conv_indent_callback($xml_string);
        if(strlen($s) >= strlen($xml_string))
            exit("strlen invalid 0");
    }

    return (microtime(true) - $t);
}


function test3(){
    global $base_iter;
    global $xml_string;
    $t = microtime(true);

    for($i = 0; $i < $base_iter; ++$i){     
        $s = conv_indent_e($xml_string);
        if(strlen($s) >= strlen($xml_string))
            exit("strlen invalid 02");
    }

    return (microtime(true) - $t);
}



echo 'XML length: ' . strlen($xml_string) . "\n";
echo 'Iterations: ' . $base_iter . "\n";

echo 'callback: ' . test0() . "\n";
echo '\G:       ' . test1() . "\n";
echo 'while:    ' . test2() . "\n";
echo '/e:       ' . test3() . "\n";


?>

首先想到以下简单的解决方案：

$xml_string = str_replace('  ', "\t", $xml_string);

但是我假设，您希望将替换限制为仅前导空格。在这种情况下，您当前的解决方案对我来说看起来很干净。也就是说，您可以在没有回调或修饰符的情况下执行此操作，但是您需要以递归方式运行它以完成如下工作：e

$re = '%# Match leading spaces following leading tabs.
    ^                     # Anchor to start of line.
    (\t*)                 # $1: Preserve any/all leading tabs.
    [ ]{2}                # Match "n" spaces.
    %umx';
while(preg_match($re, $xml_string))
    $xml_string = preg_replace($re, "$1\t", $xml_string);

令人惊讶的是，我的测试表明，这几乎是回调方法的两倍。（我猜的恰恰相反。

请注意，Qtax有一个优雅的解决方案，工作得很好（我给了它我的+1）。但是，我的基准测试显示它比原始回调方法慢。我认为这是因为表达式不允许正则表达式引擎利用：“锚定在模式开始时”内部优化。RE 引擎被迫针对目标字符串中的每个位置测试模式。对于以锚点开头的模式表达式，RE引擎只需要在每行的开头进行检查，这样它就可以更快地匹配。/(?:^|\G) /um^

好问题！+1

增编/更正：

我必须道歉，因为我上面所做的性能陈述是错误的。我只对一个（非代表性的）测试文件运行了正则表达式，该文件在前导空白处大多有选项卡。当针对具有大量前导空格的更现实的文件进行测试时，我上面的递归方法的性能明显慢于其他两种方法。

如果有人感兴趣，以下是我用来衡量每个正则表达式性能的基准脚本：

test.php

<?php // test.php 20120308_1200
require_once('inc/benchmark.inc.php');

// -------------------------------------------------------
// Test 1: Recursive method. (ridgerunner)
function tabify_leading_spaces_1($xml_string) {
    $re = '%# Match leading spaces following leading tabs.
        ^                     # Anchor to start of line.
        (\t*)                 # $1: Any/all leading tabs.
        [ ]{2}                # Match "n" spaces.
        %umx';
    while(preg_match($re, $xml_string))
        $xml_string = preg_replace($re, "$1\t", $xml_string);
    return $xml_string;
}

// -------------------------------------------------------
// Test 2: Original callback method. (hakre)
function tabify_leading_spaces_2($xml_string) {
    return preg_replace_callback('/^(?:[ ]{2})+/um', '_callback', $xml_string);
}
function _callback($m) {
    $spaces = strlen($m[0]);
    $tabs = $spaces / 2;
    return str_repeat("\t", $tabs);
}

// -------------------------------------------------------
// Test 3: Qtax's elegantly simple \G method. (Qtax)
function tabify_leading_spaces_3($xml_string) {
    return preg_replace('/(?:^|\G)  /um', "\t", $xml_string);
}

// -------------------------------------------------------
// Verify we get the same results from all methods.
$data = file_get_contents('testdata.txt');
$data1 = tabify_leading_spaces_1($data);
$data2 = tabify_leading_spaces_2($data);
$data3 = tabify_leading_spaces_3($data);
if ($data1 == $data2 && $data2 == $data3) {
    echo ("GOOD: Same results.\n");
} else {
    exit("BAD: Different results.\n");
}
// Measure and print the function execution times.
$time1 = benchmark_12('tabify_leading_spaces_1', $data, 2, true);
$time2 = benchmark_12('tabify_leading_spaces_2', $data, 2, true);
$time3 = benchmark_12('tabify_leading_spaces_3', $data, 2, true);
?>

上面的脚本使用了我前段时间写的以下方便的小基准测试函数：

benchmark.inc.php

<?php // benchmark.inc.php
/*----------------------------------------------------------------------------
 function benchmark_12($funcname, $p1, $reptime = 1.0, $verbose = true, $p2 = NULL) {}
    By: Jeff Roberson
    Created:        2010-03-17
    Last edited:    2012-03-08

Discussion:
    This function measures the time required to execute a given function by
    calling it as many times as possible within an allowed period == $reptime.
    A first pass determines a rough measurement of function execution time
    by increasing the $nreps count by a factor of 10 - (i.e. 1, 10, 100, ...),
    until an $nreps value is found which takes more than 0.01 secs to finish.
    A second pass uses the value determined in the first pass to compute the
    number of reps that can be performed within the allotted $reptime seconds.
    The second pass then measures the time required to call the function the
    computed number of times (which should take about $reptime seconds). The
    average function execution time is then computed by dividing the total
    measured elapsed time by the number of reps performed in that time, and
    then all the pertinent values are returned to the caller in an array.

    Note that this function is limited to measuring only those functions
    having either one or two arguments that are passed by value and
    not by reference. This is why the name of this function ends with "12".
    Variations of this function can be easily cloned which can have more
    than two parameters.

Parameters:
    $funcname:  String containing name of function to be measured. The
                function to be measured must take one or two parameters.
    $p1:        First argument to be passed to $funcname function.
    $reptime    Target number of seconds allowed for benchmark test.
                (float) (Default=1.0)
    $verbose    Boolean value determines if results are printed.
                (bool) (Default=true)
    $p2:        Second (optional) argument to be passed to $funcname function.
Return value:
    $result[]   Array containing measured and computed values:
    $result['funcname']     : $funcname - Name of function measured.
    $result['msg']          : $msg - String with formatted results.
    $result['nreps']        : $nreps - Number of function calls made.
    $result['time_total']   : $time - Seconds to call function $nreps times.
    $result['time_func']    : $t_func - Seconds to call function once.
    $result['result']       : $result - Last value returned by function.

Variables:
    $time:      Float epoch time (secs since 1/1/1970) or benchmark elapsed secs.
    $i:         Integer loop counter.
    $nreps      Number of times function called in benchmark measurement loops.

----------------------------------------------------------------------------*/
function benchmark_12($funcname, $p1, $reptime = 1.0, $verbose = false, $p2 = NULL) {
    if (!function_exists($funcname)) {
        exit("\n[benchmark1] Error: function \"{$funcname}()\" does not exist.\n");
    }
    if (!isset($p2)) { // Case 1: function takes one parameter ($p1).
    // Pass 1: Measure order of magnitude number of calls needed to exceed 10 milliseconds.
        for ($time = 0.0, $n = 1; $time < 0.01; $n *= 10) { // Exponentially increase $nreps.
            $time = microtime(true);            // Mark start time. (sec since 1970).
            for ($i = 0; $i < $n; ++$i) {       // Loop $n times. ($n = 1, 10, 100...)
                $result = ($funcname($p1));     // Call the function over and over...
            }
            $time = microtime(true) - $time;    // Mark stop time. Compute elapsed secs.
            $nreps = $n;                        // Number of reps just measured.
        }
        $t_func = $time / $nreps;               // Function execution time in sec (rough).
    // Pass 2: Measure time required to perform $nreps function calls (in about $reptime sec).
        if ($t_func < $reptime) {               // If pass 1 time was not pathetically slow...
            $nreps = (int)($reptime / $t_func); // Figure $nreps calls to add up to $reptime.
            $time = microtime(true);            // Mark start time. (sec since 1970).
            for ($i = 0; $i < $nreps; ++$i) {   // Loop $nreps times (should take $reptime).
                $result = ($funcname($p1));     // Call the function over and over...
            }
            $time = microtime(true) - $time;    // Mark stop time. Compute elapsed secs.
            $t_func = $time / $nreps;           // Average function execution time in sec.
        }
    } else { // Case 2: function takes two parameters ($p1 and $p2).
    // Pass 1: Measure order of magnitude number of calls needed to exceed 10 milliseconds.
        for ($time = 0.0, $n = 1; $time < 0.01; $n *= 10) { // Exponentially increase $nreps.
            $time = microtime(true);            // Mark start time. (sec since 1970).
            for ($i = 0; $i < $n; ++$i) {       // Loop $n times. ($n = 1, 10, 100...)
                $result = ($funcname($p1, $p2));     // Call the function over and over...
            }
            $time = microtime(true) - $time;    // Mark stop time. Compute elapsed secs.
            $nreps = $n;                        // Number of reps just measured.
        }
        $t_func = $time / $nreps;               // Function execution time in sec (rough).
    // Pass 2: Measure time required to perform $nreps function calls (in about $reptime sec).
        if ($t_func < $reptime) {               // If pass 1 time was not pathetically slow...
            $nreps = (int)($reptime / $t_func); // Figure $nreps calls to add up to $reptime.
            $time = microtime(true);            // Mark start time. (sec since 1970).
            for ($i = 0; $i < $nreps; ++$i) {   // Loop $nreps times (should take $reptime).
                $result = ($funcname($p1, $p2));     // Call the function over and over...
            }
            $time = microtime(true) - $time;    // Mark stop time. Compute elapsed secs.
            $t_func = $time / $nreps;           // Average function execution time in sec.
        }
    }
    $msg = sprintf("%s() Nreps:%7d  Time:%7.3f s  Function time: %.6f sec\n",
            $funcname, $nreps, $time, $t_func);
    if ($verbose) echo($msg);
    return array('funcname' => $funcname, 'msg' => $msg, 'nreps' => $nreps,
        'time_total' => $time, 'time_func' => $t_func, 'result' => $result);
}
?>

当我使用 的内容运行时，这是我得到的结果：test.phpbenchmark.inc.php

GOOD: Same results.
tabify_leading_spaces_1() Nreps: 1756 Time: 2.041 s Function time: 0.001162 sec
tabify_leading_spaces_2() Nreps: 1738 Time: 1.886 s Function time: 0.001085 sec
tabify_leading_spaces_3() Nreps: 2161 Time: 2.044 s Function time: 0.000946 sec

底线：我建议使用Qtax的方法。

感谢Qtax！