正则表达式 - 排除字符串的负前瞻

Question

I am trying to find (and replace with something else) in a text all parts which

1. start with '/'
2. ends with '/'
3. between the two /'s there can be anything, except the strings '.' and '..'.

(For your info, I am searching for and replacing directory and file names, hence the '.' and '..' should be excluded.)

This is the regular expression I came up with:

/(?!\.|\.\.)([^/]+)/

The second part

([^/]+)

matches every sequence of characters, '/' excluded. There are no character restrictions required, I am simply interpreting the input.

The first part

(?!\.|\.\.)

uses the negative lookahead assertion to exclude the strings '.' and '..'.

However, this doesn't seem to work in PHP with mb_ereg_replace().

Can somebody help me out? I fail to see what's wrong with my regex.

Thank you.

Answer 1

POSIX regex probably don't have support for negative lookaheads. (I may be wrong though)

Anyway since PCRE regex are usually faster than POSIX I think you can use PCRE version of the same function since PCRE supports utf8 as well using u flag.

Consider this code as a substitute:

preg_replace('~/(?!\.|\.\.)([^/]+)/~u', "", $str);

EDIT: Even better is to use:

preg_replace('~/(?!\.)([^/]+)/~u', "", $str);

Answer 2

This is a little verbose, but it definitely does work:

#/((\.[^./][^/]*)|(\.\.[^/]+)|([^.][^/]*))/#
^  |------------| |---------| |---------|
|        |             |               |
|        |        text starting with   |
|        |        two dots, that isn't |
|        |             "." or ".."     |
|  text starting with                  |
|  a dot, that isn't                text not starting
|  "." or ".."                         with a dot
|
delimiter

Does not match:

• hi
• //
• /./
• /../

Does match:

• /hi/
• /.hi/
• /..hi/
• /... /

Have a play around with it on http://regexpal.com/ .

I wasn't sure whether or not you wanted to allow // . If you do, stick * before the last / .

Answer 3

I'm not against regex, but I would have done this instead:

function simplify_path($path, $directory_separator = "/", $equivalent = true){
  $path = trim($path);
  // if it's absolute, it stays absolute:
  $prepend = (substr($path,0,1) == $directory_separator)?$directory_separator:"";
  $path_array = explode($directory_separator, $path);
  if($prepend) array_shift($path_array);
  $output = array();
  foreach($path_array as $val){
    if($val != '..' || ((empty($output) || $last == '..') && $equivalent)) {
      if($val != '' && $val != '.'){
        array_push($output, $val);
        $last = $val;
      }
    } elseif(!empty($output)) {
        array_pop($output);
    }
  }
  return $prepend.implode($directory_separator,$output);
}

---

Tests:

echo(simplify_path("../../../one/no/no/../../two/no/../three"));
// =>  ../../../one/two/three
echo(simplify_path("/../../one/no/no/../../two/no/../three"));
// =>  /../../one/two/three
echo(simplify_path("/one/no/no/../../two/no/../three"));
// =>  /one/two/three
echo(simplify_path(".././../../one/././no/./no/../../two/no/../three"));
// =>  ../../../one/two/three
echo(simplify_path(".././..///../one/.///./no/./no/../../two/no/../three/"));
// =>  ../../../one/two/three

I thought that it would be better to return an equivalent string, so I respected the ocurrences of .. at the begining of the string.

If you dont want them, you can call it with the third parameter $equivalent = false:

echo(simplify_path("../../../one/no/no/../../two/no/../three", "/", false));
// =>  one/two/three
echo(simplify_path("/../../one/no/no/../../two/no/../three", "/", false));
// =>  /one/two/three
echo(simplify_path("/one/no/no/../../two/no/../three", "/", false));
// =>  /one/two/three
echo(simplify_path(".././../../one/././no/./no/../../two/no/../three", "/", false));
// =>  one/two/three
echo(simplify_path(".././..///../one/.///./no/./no/../../two/no/../three/", "/", false));
// =>  one/two/three

Answer 4

/(?.(\.|\.\.)/)([^/]+)/ This will allow ... as a valid name.