If you just want to convert '&' in content and avoid tags (ie: attribute values)
and avoid script blocks, something like below will work for most occurances.
However, it should be noted that attribute values should be converted as well.
And doing that requires a lot more work.

Working sample http://www.ideone.com/9MhCq

<?php

$html=<<<EOD
 <some &ta&g> S&P &&more; and &some; <more> &notme;
  && &#209; &#xa92F;
 <script flavor?>
   val && this & this 
 </script>
 &
EOD;

$rxent = '(?:&(?:[A-Za-z_:][\w:.-]*|\#(?:[0-9]+|x[0-9a-fA-F]+));)';

$rxtag =
'<
 (?:
     \?php\s+.*?\?
  |  (?:
       (?:
           (?:script|style)\s*
         | (?:script|style)\s+(?:".*?"|\'.*?\'|[^>]*?)+\s*
       )> .*? </(?:script|style)\s*
     )
  |  (?:
         /?[A-Za-z_:][\w:.-]*\s*/?
       |  [A-Za-z_:][\w:.-]*\s+(?:".*?"|\'.*?\'|[^>]*?)+\s*/?
       | !(?:DOCTYPE.*?|--.*?--)
     )
 )
 >
';

$rxmain = "~(?xs:((?:$rxtag)+) | ((?!$rxent)&))~";


print "$html\n\n";

$html = preg_replace_callback($rxmain, 'fixamp_cb', $html);

print "$html\n";

function fixamp_cb( $matches ) {
    # Return tags and script blocks unchanged.
    if (isset($matches[1]) && $matches[1])
       return $matches[1];
    return '&amp;';
}

?>

Actually you need to escape $ character by a backslash \ in your pattern. Also to make it more confident, you can add \s* which means zero or more white space(s) before = character.

Use this pattern:

/\$search(?!\s*\=)/i

Online Demo

• / delimiter
• \$ matches $ literally
• search matches that word literally
• (?! negative Lookahead which checks being both white space(s) (if exists) and = after that variable name.
• /i modifire makes the pattern insensitive (there isn't any different between a-z and A-Z)

Note: You have to use single quotes '/pattern/' for pattern above. If you want to use double quotes " then you should escape backslashes again:

/\\$search(?!\\s*\\=)/i

You may enforce the presence of a letter, digit, ., _ or - by removing the ? quantifier after / inside the quantified capturing group. Also, you may use \w instead of [a-zA-Z0-9_] to shorten the pattern a bit. A non-capturing group will also be more appropriate since you do not need to capture the value of a repeated capturing group (replace ( with (?:).

Use

/^\/page(?:\/[\w.-]+)*\/?$/i
            ^ 

See the regex demo.

If you're looking to compare the version, you can strip on the space and then use version_compare().

If you just want the numeric representation, use a regex to simply use preg_replace() all non digits in the original version string.

$version = '2.3.3 Release';
echo preg_replace('/\D+/', '', $version);

I suggest this regex that doesn't require any anchoring (thus can be used on a page containing links and text):

<a href\s?=\s?"http(s)?://([^"]+)">([^<]+)</a>|(http)?(s)?(://)?((?:[-\w]+\.)+\S+[^,.\s])

Usage:

$result = preg_replace('~<a href\s?=\s?"http(s)?://([^"]+)">([^<]+)</a>|(http)?(s)?(://)?((?:[-\w]+\.)+\S+[^,.\s])~', '<a href="http$1$5://$2$7">$3$4$5$6$7</a>', $text);

The regex matches both the 'normal' urls and the ones that are already wrapped between anchor tags, but treats them differently in the replace.

You mixed lookahead and lookbehind positions.

The (?![\'"])(\w+) is equal to (\w+) because (?![\'"]) is a negative lookahead and requires the next char not to be a ' or ", but since the next pattern is \w, matching a word char, the lookahead becomes redundant. You need to use a negative lookbehind here, (?<![\'"]) (\w+). And the problem with (\w+)(?<![\'"]) is similar: the word char cannot be a ' and " and the negative lookbehind is redundant. You wanted a lookahead here.

You need to use

'~(?:^|\b) (?<![\'"]) (\w+) (?![\'"]) :~mx'

See the regex demo.

I my opinion you don't need lookahead at all. Try this:

$find = array(
    '~\[url=([^]]+)]\img\[/img]\[/url]~i'
);

$replace = array(
    '<a href="$1" target="_blank"><img src="$2" class="bbcodeimage img-polaroid" alt="[img]" /></a>'
);

Explanations:

First at all, I have changed the pattern delimiter to ~, the goal of this change is to avoid to escape all literal / in the pattern. Literals ] don't need to be escaped outside a character class or inside a character class if (and only if) it is the first character.

A lookahead is not useful in this situation because a lookahead is only a check and matches nothing. Example a(?=bc) will find a a followed by bc but will only match the a. It is why lookaheads and lookbehinds are also called "zero width assertions".

pattern details:

~           # delimiter
\[url=      # literal: [url=
(           # open the first capturing group
    [^]]+   # all characters except ] (one or more times)
)           # close the first capturing group
]           # literal: ]
\[img]      # literal: [img]
(           # open the second capturing group
    [^[]+   # all characters except [ (one or more times)
)           # close the second capturing group
\[/img]     # literal: [/img]
\[/url]     # literal: [/url]
~i          # delimiter and i modifier

Note that I have choosen to use single quotes for the replacement string to avoid to escape all double quotes of the string (and because there is no reason to use double quotes, no variables, no \n or \t etc.).

The problem is your regex is matching /plugins/ - matching slashes at both the front and the end. Each letter is only matched by the regex once, so if a slash is matched at the end of one word it can't also be counted as the start of another. Hence, it's only matching every other one.

Try this instead:

<?php
    $pattern        = '#(/)([a-z\_]+)(?=/)#'; 
    $replacement    = '$1{char}'; 
    $string         = '/plugins/plugins/plugins/plugins/plugins/plugins/';

    echo preg_replace($pattern, $replacement, $string);
?>

It works by using lookahead, instead of actually matching the final slash (and "consuming" it) it just checks to make sure it's there.