title: Search specified string inside textbox
body: <p>i have textbox with text:</p>
  
  <pre><code>1234 YYMM 1057316895 12, AB 6386 ABC
  </code></pre>
  
  <p>where YYMM is in this case Year and Month. What i'd like to do is to search if in this textbox exist YYMM, and highlight this part of text, or somehow show that in this specified textbox exist not fully completed field. </p>
  
  <p>So when i rewrite this string with 1203 instead of YYMM error will not be received.</p>
  
  <p>And! This YYMM can be in any place of string in textbox, so i can't do something like</p>
  
  <pre><code>if (textbox1.Text.Substring(x,4)=="YYMM) {}
  </code></pre>
  
  <p>where x is index of YYMM location. </p>
  
  <p>Tnx</p>