Efficient way to get last part of string only after a certain base string

Question

What is an efficient way to get the part of a string after the occurrence of a certain needle, only if that needle is at the start of the haystack. Similar to strstr(), but excluding the needle, and only when found at the beginning of the string.

If it isn't found, it should preferably return false.

I have the feeling I'm overlooking some very obvious PHP functions here, but I can't seem to think of them right now.

For example:

$basePath = '/some/dir/';

$result = some_function( '/some/dir/this/is/the/relative/part', $basePath );
/*
should return:
this/is/the/relative/part
*/

$result = some_function( '/fake/dir/this/is/the/relative/part', $basePath );
$result = some_function( '/pre/some/dir/this/is/the/relative/part', $basePath );
/*
last example has '/some/dir/' in it, but not at start.
should both preferably return:
false
*/

I'll be using this for a filesystem service that should act as a sandbox, and should be able to give out and take in paths, relative to the base sand box directory.

mario · Accepted Answer · 2011-03-17 16:49:40Z

3

This case calls for strncmp:

function some_function($path, $base) {
    if (strncmp($path, $base, $n = strlen($base)) == 0) {
        return substr($path, $n);
    }
}

answered Mar 17, 2011 at 16:49

mario

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1 Comment

Decent Dabbler Over a year ago

Yeah, I think this is the way to go. Thanks. Brad was a bit earlier with a very similar strncmp variant. But this one is a tad bit sleeker though. :D

Brad Christie · Accepted Answer · 2011-03-17 16:43:38Z

1

Put more simply than the other examples:

function some_function($path,$base){
  $baselen = strlen($base);
  if (strpos($path,$base) === 0 && strlen($path)>$baselen)
    return substr($path,$baselen);
  return false;
}

DEMO

Alternate using strncmp, too: DEMO

answered Mar 17, 2011 at 16:43

Brad Christie

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5 Comments

Decent Dabbler Over a year ago

I like the strncmp variant very much!

Decent Dabbler Over a year ago

PS.: the strlen($path) > $baselen on the strncmp variant is redundant, no? Or am I overlooking something vital here?

Brad Christie Over a year ago

@fireeyedboy: I added the check in case the path matches the base. You'll receive a false back saying, basically, "i have no further information to grab".

Decent Dabbler Over a year ago

Ah yes of course, clever! Although, in this case I don't mind it returning an empty string. As this would be a valid relative path. Good thinking though. Much brighter than I am today. :)

Decent Dabbler Over a year ago

About your strncasecomp() remark. Good point! But seeing I'm dense today, I'll have to ponder on the implications of this first, before commenting any more. :) Thanks so far!

xn. · Accepted Answer · 2011-03-17 16:54:55Z

1

function some_function($haystack, $baseNeedle) {
   if (! preg_match('/^' . preg_quote($baseNeedle, '/') . '(.*)$/', $haystack, $matches)) {
      return false;
   }
   return $matches[1];
}

answered Mar 17, 2011 at 16:54

xn.

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Comments

erisco · Accepted Answer · 2011-03-17 16:40:57Z

0

function some_function($path, $basePath) {
    $pos = strpos($path, $basePath);
    if ($pos !== false) {
        return substr($path, strlen($basePath));
    } else {
        return false;
    }
}

But what about this?

some_function('/base/path/../../secret/password.txt', '/base/path/safe/dir/');

You likely want to call realpath() on $path first, so that it is fully simplified, before using some_function().

answered Mar 17, 2011 at 16:40

erisco

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6 Comments

Decent Dabbler Over a year ago

Good point about realpath(), but I'm using that already actually. Thanks!

Brad Christie Over a year ago

@fireeyedboy: Though it's really on valid in a *nix environment. ;-) (Unless you switch to strncasecmp)

Decent Dabbler Over a year ago

@Brad: not sure I follow you. Sorry, I'm a bit dense today. Do you mean to say realpath() is only valid on *nix? Because of case sensitivity perhaps?

Brad Christie Over a year ago

@fireeyedboy: Actually, I'm the one slipping. :grin: The answers [min & erisco] were right below each other. I saw your reply to my answer and though I was replying back in the same post. D'oh! My original comment was in regards to *nix being case sensitive with paths (your reference to strncmp and my reply with strncasecmp). My apologies for the confusion.

Decent Dabbler Over a year ago

@erisco: if ($pos !== false) should actually be if ($pos === 0) in my case, because it should only be valid if the needle is found at the start of the haystack.

|

Czechnology · Accepted Answer · 2011-03-17 16:42:28Z

0

Not sure if there's a built-in function for this.

function some_function($str1, $str2) {
  $l = strlen($str2);
  if (substr($str1, 0, $l) == $str2)
    return substr($str1, $l);
  else
    return false;
}

For long $str1 and short $str2, I think this is going to be faster than using strpos.

If you're using this for paths, you might want to do some checking if the slashes are at the right place.

answered Mar 17, 2011 at 16:42

Czechnology

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Comments

bart · Accepted Answer · 2011-03-17 20:19:59Z

0

How about regular expressions?

$regexp = preg_quote($basepath, '/');
return preg_replace("/^$regexp/", "", $path);

answered Mar 17, 2011 at 20:19

bart

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