PHP : Strings - [1.6] Expanding and Compressing Tabs

1.6.1 Problem

You want to change spaces to tabs (or tabs to spaces) in a string while keeping text aligned with tab stops. For example, you want to display formatted text to users in a standardized way.

1.6.2 Solution

Use str_replace( ) to switch spaces to tabs or tabs to spaces:

$r = mysql_query("SELECT message FROM messages WHERE id = 1") or die();
$ob = mysql_fetch_object($r);
$tabbed = str_replace(' ',"\t",$ob->message);
$spaced = str_replace("\t",' ',$ob->message);

print "With Tabs: <pre>$tabbed</pre>";
print "With Spaces: <pre>$spaced</pre>";

Using str_replace( ) for conversion, however, doesn't respect tab stops. If you want tab stops every eight characters, a line beginning with a five-letter word and a tab should have that tab replaced with three spaces, not one. Use the pc_tab_expand( ) function shown in Example 1-1 to turn tabs to spaces in a way that respects tab stops.

Example 1-1. pc_tab_expand( )

function pc_tab_expand($a) {
  $tab_stop = 8;
  while (strstr($a,"\t")) {
    $a = preg_replace('/^([^\t]*)(\t+)/e',
                      "'\\1'.str_repeat(' ',strlen('\\2') * 
                       $tab_stop - strlen('\\1') % $tab_stop)",$a);
  } 
  return $a;
}

$spaced = pc_tab_expand($ob->message);

You can use the pc_tab_unexpand() function shown in Example 1-2 to turn spaces back to tabs.

Example 1-2. pc_tab_unexpand( )

function pc_tab_unexpand($x) {
  $tab_stop = 8;
  
  $lines = explode("\n",$x);
  for ($i = 0, $j = count($lines); $i < $j; $i++) {
    $lines[$i] = pc_tab_expand($lines[$i]);
    $e = preg_split("/(.\{$tab_stop})/",$lines[$i],-1,PREG_SPLIT_DELIM_CAPTURE);
    $lastbit = array_pop($e);
    if (!isset($lastbit)) { $lastbit = ''; }
    if ($lastbit == str_repeat(' ',$tab_stop)) { $lastbit = "\t"; }
    for ($m = 0, $n = count($e); $m < $n; $m++) {
      $e[$m] = preg_replace('/  +$',"\t",$e[$m]);
    }
    $lines[$i] = join('',$e).$lastbit;
  }
  $x = join("\n", $lines);
  return $x;
}

$tabbed = pc_tab_unexpand($ob->message);

Both functions take a string as an argument and return the string appropriately modified.

1.6.3 Discussion

Each function assumes tab stops are every eight spaces, but that can be modified by changing the setting of the $tab_stop variable.

The regular expression in pc_tab_expand( ) matches both a group of tabs and all the text in a line before that group of tabs. It needs to match the text before the tabs because the length of that text affects how many spaces the tabs should be replaced so that subsequent text is aligned with the next tab stop. The function doesn't just replace each tab with eight spaces; it adjusts text after tabs to line up with tab stops.

Similarly, pc_tab_unexpand( ) doesn't just look for eight consecutive spaces and then replace them with one tab character. It divides up each line into eight-character chunks and then substitutes ending whitespace in those chunks (at least two spaces) with tabs. This not only preserves text alignment with tab stops; it also saves space in the string.