title: remove characters from a string in a data frame
body: <p>I have a data frame where column "ID" has values like these:
  1234567_GSM00298873
  1238416_GSM90473673
  98377829</p>
  
  <p>In other words, some rows have 7 numbers followed by "_" followed by letters and numbers; other rows have just numbers</p>
  
  <p>I want to remove the numbers and the underscore preceding the letters, without affecting the rows that have only number. I tried</p>
  
  <pre><code>dataframe$ID &lt;- gsub("*_", "", dataframe$ID)
  </code></pre>
  
  <p>but that only removes the underscore. So I learned that * means zero or more. 
  Is there a wildcard, and a repetition operator such that I can tell it to find the pattern "anything-seven-times-followed-by-_"?
  Thanks!</p>