title: Regex Valid Twitter Mention
body: <p>I'm trying to find a regex that matches if a Tweet it's a true mention. To be a mention, the string can't start with "@" and can't contain "RT" (case insensitive) and "@" must start the word.</p>
  
  <p>In the examples I commented the desired output</p>
  
  <p>Some examples:</p>
  
  <pre><code>function search($strings, $regexp) {
      $regexp;
      foreach ($strings as $string) {
          echo "Sentence: \"$string\" &lt;- " .
          (preg_match($regexp, $string) ? "MATCH" : "NO MATCH") . "\n";
      }
  }
  
  $strings = array(
  "Hi @peter, I like your car ", // &lt;- MATCH
  "@peter I don't think so!", //&lt;- NO MATCH: the string it's starting with @ it's a reply
  "Helo!! :@ how are you!", // NO MATCH &lt;- it's not a word, we need @(word) 
  "Yes @peter i'll eat them this evening! RT @peter: hey @you, do you want your pancakes?", // &lt;- NO MATCH "RT/rt" on the string , it's a RT
  "Helo!! ineed@aser.com how are you!", //&lt;- NO MATCH, it doesn't start with @
  "@peter is the best friend you could imagine. RT @juliet: @you do you know if @peter it's awesome?" // &lt;- NO MATCH starting with @ it's a reply and RT
  );
  echo "Example 1:\n";
  search($strings,  "/(?:[[:space:]]|^)@/i");
  </code></pre>
  
  <p>Current output:</p>
  
  <pre><code>Example 1:
  Sentence: "Hi @peter, I like your car " &lt;- MATCH
  Sentence: "@peter I don't think so!" &lt;- MATCH
  Sentence: "Helo!! :@ how are you!" &lt;- NO MATCH
  Sentence: "Yes @peter i'll eat them this evening! RT @peter: hey @you, do you want your pancakes?" &lt;- MATCH
  Sentence: "Helo!! ineed@aser.com how are you!" &lt;- MATCH
  Sentence: "@peter is the best friend you could imagine. RT @juliet: @you do you know if @peter it's awesome?" &lt;- MATCH
  </code></pre>
  
  <p>EDIT:</p>
  
  <blockquote>
    <p>I need it in regex beacause it can be used on MySQL and anothers
    languages too. Im am not looking for any username. I only want to know
    if the string it's a mention or not.</p>
  </blockquote>