preg_replace中替换数组中的特殊（转义）字符会被转义

Question

I’m trying to modify a string of the following form where each field is delimited by a tab except for the first which is followed by two or more tabs.

"$str1      $str2   $str3   $str4   $str5   $str6"

The modified string will have each field wrapped in HTML table tags, and be on its own, indented line as so.

"<tr>
  <td class="title">$str1</td>
  <td sorttable_customkey="$str2"></td>
  <td sorttable_customkey="$str3"></td>
  <td sorttable_customkey="$str4"></td>
  <td sorttable_customkey="$str5"></td>
  <td sorttable_customkey="$str6"></td>
</tr>

"

I tried using code like the following to do it.

$patterns = array();
$patterns[0]='/^/';
$patterns[1]='/\t\t+/';
$patterns[2]='/\t/';
$patterns[3]='/$/';

$replacements = array();
$replacements[0]='\t\t<tr>
\t\t\t<td class="title">';
$replacements[1]='</td>
\t\t\t<td sorttable_customkey="';
$replacements[2]='"></td>
\t\t\t<td sorttable_customkey="';
$replacements[3]='"></td>
\t\t</tr>
';

for ($i=0; $i<count($lines); $i++) {
  $lines[$i] = preg_replace($patterns, $replacements, $lines[$i]);
}

The problem is that the escaped characters (tabs and newlines) in the replacement array remain escaped in the destination string and I get the following string.

"\t\t<tr>
\t\t\t<td class="title">$str</td>
\t\t\t<td sorttable_customkey="$str2"></td>
\t\t\t<td sorttable_customkey="$str3"></td>
\t\t\t<td sorttable_customkey="$str4"></td>
\t\t\t<td sorttable_customkey="$str5"></td>
\t\t\t<td sorttable_customkey="$str6"></td>
\t\t</tr>
"

Strangely, this line I tried earlier on does work:

$data=preg_replace("/\t+/", "\t", $data);

Am I missing something? Any idea how to fix it?

Answer 1

You need double quotes or heredocs for the replacement string - PCRE only parses those escape characters in the search string.

In your working example preg_replace("/\t+/", "\t", $data) those are both literal tab characters because they're in double quotes.

If you changed it to preg_replace('/\t+/', '\t', $data) you can observe your main problem - PCRE understands that the \t in the search string represents a tab, but doesn't for the one in the replacement string.

So by using double quotes for the replacement, e.g. preg_replace('/\t+/', "\t", $data), you let PHP parse the \t and you get the expected result.

It is slightly incongruous, just something to remember.