title: Filter lines that have only alphabets in first column
body: <p>I am working with google english 1gram dataset <a href="http://storage.googleapis.com/books/ngrams/books/googlebooks-eng-all-1gram-20120701-c.gz" rel="nofollow">link here</a>, it looks like the following:</p>
  
  <pre><code>C'ape   1804    1       1
  C'ape   1821    1       1
  C'ape   1826    1       1
  C'ape   1838    2       2
  C'ape   1844    1       1
  C'ape   1869    1       1
  C'ape   1874    1       1
  C'ape   1878    2       2
  C'ape   1879    1       1
  C'ape   1880    1       1
  CABMEL  1873    1       1
  CABMEL  1874    1       1
  CABMEL  1875    1       1
  CABMEL  1879    1       1
  CABMEL  1884    1       1
  CABMEL  1890    1       1
  CABMEL  1899    1       1
  CABMEL  1901    1       1
  CABMEL  1903    3       2
  CABMEL  1910    2       2
  CABMEL  1912    1       1
  CABMEL  1915    1       1
  CABMEL  1926    2       2
  CABMEL  1927    3       2
  CABMEL  1928    4       2
  CABMEL  1930    2       2
  </code></pre>
  
  <p>At least 4 columns, and some rows also contain 5. First column is a 1-gram, a string, I want to extract only those lines which have a string in first column that only contains letters (upper case or lower case alphabets only). I am thinking grep should do it but I cannot find the correct regex to do this job. Any unix utilty that can easily get the job done?
  Columns are tab delimited I believe.</p>
  
  <p>EDIT: Output will contain only the lines with CABMEL</p>