Question

I have some XML chunk returned by DOMDocument::saveXML(). It’s already pretty indented, with two spaces per level, like so:

<?xml version="1.0"?>
<root>
  <error>
    <a>eee</a>
    <b>sd</b>
  </error>
</root>

As it’s not possible to configure DOMDocument (AFAIK) about the indentation character(s), I thought it’s possible to run a regular expression and change the indentation by replacing all two-space-pairs into a tab. This can be done with a callback function (Demo):

$xml_string = $doc->saveXML();
function callback($m)
{
    $spaces = strlen($m[0]);
    $tabs = $spaces / 2;
    return str_repeat("\t", $tabs);
}
$xml_string = preg_replace_callback('/^(?:[ ]{2})+/um', 'callback', $xml_string);

I’m now wondering if it’s possible to do this w/o a callback function (and without the e-modifier (EVAL)). Any regex wizards with an idea?

Answer 1

You can use \G:

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

---

Did some benchmarks and got following results on Win32 with PHP 5.2 and 5.4:

>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

Surprising that callback is faster than than \G in PHP 5.4 (altho that seems to depend on the data, \G is faster in some other cases).

For \G /^ |\G /m is used, and is a bit faster than /(?:^|\G) /m.
/(?>^|\G) /m is even slower than /(?:^|\G) /m.
/u, /S, /X switches didn’t affect \G performance noticeably.

The while replace is fastest if depth is low (up to about 4 indentations, 8 spaces, in my test), but then gets slower as the depth increases.

The following code was used:

<?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";


?>