title: Why does expr match outputs different things when the pattern (not) contains a parenthesized group?
body: <p>Why does </p>
  
  <pre><code>$ echo `expr match abcdef 'abc'`
  </code></pre>
  
  <p>give number of characters matched, which is 3, but</p>
  
  <pre><code>$ echo `expr match abcdef '\(abc\)'`
  </code></pre>
  
  <p>gives the characters matched , which is abc ?</p>
  
  <p>I understand regex matching is in play here, but cannot understand how having a parenthesized sub expression is making this difference here?</p>