The [[:blank:]] represents horizontal space, tab-space characters in regular expression.

    <?php
    $str = 'A12 34ax$ xxxx';
    $ret = ereg_replace("[[:upper:]]|[[:blank:]]","B",$str);
    echo $ret;
    ?>

     B12B34ax$Bxxxx

In the above example for regex the character class "[[:blank:]]" is used along with "[[:upper:]]" to match a string that is in uppercase and having one or more tab or blank-space.

    #! C:\programfiles\perl\bin\perl
    print "content-type: text/html\n\n";
    $str = "A A AA";
    if ( $str =~ /[[:blank:]]{1,2}/ ) 
      {print "Matches!";} 
    else 
      {print "Unmatch!";}

     Matches!

In the above example for regex the string "A A AA" has a two blank-spaces, so does tha pattern "[[:blank:]] {1,2}" which specifies number of blank-spaces minimum "1" space and utmost "2" blank-spaces.