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An unexpected output caused by misunderstood non-greedy mode of regular expression in PHP

2013-04-12 00:02 615 查看

Recently I've been working on a PHP project with my old small framework, parts of which, including the template engine, are from Discuz!. An unexpected output from the template engine surprised me.

<?php

$input = '<html>
<table>
<thead>{loop $thread $head}<td>$head</td>{/loop}</thead>
{loop $list2 $node}
<tr>$node</tr>
{/loop}
</table>';

$output = preg_replace('/\{loop\s+(\S+)\s+(\S+)\}[\r\n\t]*(.+?)[\r\n\t]*\{\/loop\}/is', '<?php foreach(\\1 as \\2){?>\\3<?php }?>', $input);

echo htmlspecialchars($output);

$input = '<html>
<table>
<thead>{loop $thread $head}<td>$head</td>{/loop}</thead>
{loop $list2 $node}
<tr>$node</tr>
{/loop}
</table>';

echo '<br />';

$output = preg_replace('/\{loop\s+(\S+)\s+(\S+)\}[\r\n\t]*(.+)[\r\n\t]*\{\/loop\}/is', '<?php foreach(\\1 as \\2){?>\\3<?php }?>', $input);

echo htmlspecialchars($output);

?>

The situation was just like the code above. It's noticed that a non-greedy operator "?" is expected to match the first {loop $thread $head} and the first {/loop}. But unfortunately it matches the first {loop $thread $head} and the second {/loop}. Non-greedy
mode seems not working. Because output1 and output2 are exactly the same.

Finally I found my solution as follows.

<?php

$input = '<html>
<table>
<thead>
{loop $thread $head}
<td>$head</td>
{/loop}
</thead>
{loop $list2 $node}
<tr>$node</tr>
{/loop}
</table>';

$output = preg_replace('/\{loop\s+(\S+)\s+(\S+)\}[\r\n\t]*(.+?)[\r\n\t]*\{\/loop\}/is', '<?php foreach(\\1 as \\2){?>\\3<?php }?>', $input);

echo htmlspecialchars($output);

?>

The non-greedy mode is kind of disordered because of the following [\r\n\t]*, which was what I didn't notice before. The regular expression engine tries to match as many as subexpressions.

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