title: R - test if first occurrence of string1 is followed by string2
body: <p>I have an R string, with the format</p>
  
  <pre><code>s = `"[some letters and numbers]_[a number]_[more numbers, letters, punctuation, etc, anything]"`
  </code></pre>
  
  <p>I simply want a way of checking if <code>s</code> contains <code>"_2"</code> in the first position. In other words, after the first <code>_</code> symbol, is the single number a "2"? How do I do this in R?</p>
  
  <p>I'm assuming I need some complicated regex expresion?</p>
  
  <p>Examples:</p>
  
  <pre><code>39820432_2_349802j_32hfh = TRUE
  </code></pre>
  
  <p><code>43lda821_9_428fj_2f = FALSE</code> (notice there is a <code>_2</code> there, but not in the right spot)</p>