title: Vim Syntax: Matching a regex in a region to after AND before a keyword
body: <p>I am looking for a way to code a <code>.vim</code> syntax script to match variable names inside a region in a flexible way.</p>
  
  <p>The syntax of the portion of the language I am trying to highlight is like so:</p>
  
  <pre><code>Function(arg1, arg2, ... argn);
  </code></pre>
  
  <p>where <code>args</code> are of the form <code>infoo=outfoo</code>.</p>
  
  <p>Whitespace is ignored in this language.
  The code I have so far indicates a region <code>parens</code></p>
  
  <pre><code>syn region parens start='(' end=')'
  </code></pre>
  
  <p>Is there a way to match regex expressions using the <code>syn match 'regex'</code> command to before and after the <code>=</code> character within the <code>parens</code> region only?</p>
  
  <p>The problem is more complicated than simply matching all words in the <code>parens</code> region because I need to highlight <code>infoo</code> and <code>outfoo</code>, except when one of them is already something I have implemented syntax highlighting for. Said another way, <code>infoo</code> and <code>outfoo</code> must be different colors depending on whether they are declared as variable names or are hardcoded into the language.</p>
  
  <p>I have followed <a href="http://vim.wikia.com/wiki/Creating_your_own_syntax_files" rel="nofollow">the Vim wiki guide</a> for most of my efforts, and I am finding it difficult to understand the Vim <a href="http://vimdoc.sourceforge.net/htmldoc/syntax.html" rel="nofollow">documentation</a>. This is my first attempt at writing or editing Vim syntax highlighting</p>