title: I need a fast runtime expression parser
body: <p>I need to locate a fast, lightweight expression parser.</p>
  
  <p>Ideally I want to pass it a list of name/value pairs (e.g. variables) and a string containing the expression to evaluate.  All I need back from it is a true/false value.  </p>
  
  <p>The types of expressions should be along the lines of:</p>
  
  <pre><code>varA == "xyz" and varB==123
  </code></pre>
  
  <p>Basically, just a simple logic engine whose expression is provided at runtime.</p>
  
  <p><strong>UPDATE</strong><br>
  At minimum it needs to support ==, !=, >, >=, &lt;, &lt;=</p>
  
  <p>Regarding speed, I expect roughly 5 expressions to be executed per request.  We'll see somewhere in the vicinity of 100/requests a second.  Our current pages tend to execute in under 50ms.  Usually there will only be 2 or 3 variables involved in any expression.  However, I'll need to load approximately 30 into the parser prior to execution.</p>
  
  <p><strong>UPDATE 2012/11/5</strong><br>
  Update about performance.  We implemented nCalc nearly 2 years ago.  Since then we've expanded it's use such that we average 40+ expressions covering 300+ variables on post backs.  There are now thousands of post backs occurring per second with absolutely zero performance degradation.  </p>
  
  <p>We've also extended it to include a handful of additional functions, again with no performance loss.  In short, <a href="http://ncalc.codeplex.com/" rel="noreferrer">nCalc</a> met all of our needs and exceeded our expectations.</p>