title: as.numeric returns NA for no apparent reason for some of the values in a column
body: <p>While trying to convert a column of characters (strings of numbers, e.g "0.1234") into numeric, using as.numeric, some of the values are returned <code>NA</code> with the warning 'NAs introduced by coercion'. The characters that are returned as <code>NA</code>s don't seem to be any different from the ones that are returned as numeric correctly. Does anyone know what can be the problem?</p>
  
  <p>Already tried to look for any characters that are not numeric (as ',') that can hide inside some of the values. I did find strings containing '-' (e.g "-0.123") that really turned into <code>NA</code>s, but these are only part of the strings turned into <code>NA</code>s. Also, tried to look for spaces inside the strings. that doesn't seem to be the problem as well. </p>
  
  <pre><code>data$y
   [1] "0.833250539"  "0.820323535"  "0.462284612"  "0.792943985"  "0.860587952"  "0.729665177"  "0.461503956"  "0.625871118" 
   [9] "0.740999346"  "0.962727964"  "0.971089266"  "0.869004848"  "0.828651766"  "0.900648732"  "0.970326033"  "0.898123286" 
  [17] "0.911640765"  "0.902442126"  "0.843392097"  "0.763421844"  "0.892426243"  "0.380433624"  "0.925017633"  "0.725470821" 
  [25] "0.699924767"  "0.689061225"  "0.907462936"  "0.888064239"  "0.913547115"  "-‬0.625103904‭" "0.897385961"  "0.889727462" 
  [33] "0.90127339"   "0.947012474"  "0.948883588"  "0.845845512"  "0.97866966"   "0.796247738"  "0.864627056"  "0.266656189‭" 
  [41] "0.894915463"  "0.969690678"  "0.771365656‭"  "0.88304436"   "0.954039006"  "0.836952199"  "0.731558669‭"  "0.907224294" 
  [49] "0.622059127"  "0.887742343"  "0.917550343"  "0.97240334‭"   "0.902841957"  "0.617403052"  "0.82926708"   "0.674903846" 
  [57] "0.947132958"  "0.929213613‭"  "-‬0.297844476" "0.871767367"
  
  y = as.numeric(data$y)
  </code></pre>
  
  <blockquote>
    <p>Warning message:
    NAs introduced by coercion </p>
  </blockquote>
  
  <pre><code>y
   [1] 0.8332505 0.8203235 0.4622846 0.7929440 0.8605880 0.7296652 0.4615040 0.6258711 0.7409993 0.9627280 0.9710893 0.8690048 0.8286518
  [14] 0.9006487 0.9703260 0.8981233 0.9116408 0.9024421 0.8433921 0.7634218 0.8924262 0.3804336 0.9250176 0.7254708 0.6999248 0.6890612
  [27] 0.9074629 0.8880642 0.9135471        NA 0.8973860 0.8897275 0.9012734 0.9470125 0.9488836 0.8458455 0.9786697 0.7962477 0.8646271
  [40]        NA 0.8949155 0.9696907        NA 0.8830444 0.9540390 0.8369522        NA 0.9072243 0.6220591 0.8877423 0.9175503        NA
  [53] 0.9028420 0.6174031 0.8292671 0.6749038 0.9471330        NA        NA 0.8717674
  </code></pre>