title: the difference between En_US and en_US?
body: <p>today I found a question in sorting a file with linux sort command. When I set the env LANG=En_US, the result is what I expect. But when LANG=en_US, the result is strange.
  Some commands I ran and the output as follows:</p>
  
  <pre><code>[work@xx:/data1/muce_temp/datamarts/reduce_result_file/302/1d/201212260000]$ cat dd.dat                 
  23 340_guard    16                                                                                                        
  23 340_guard    17                                                                                                        
  23 340_guard    18                                                                                                        
  23 360_guard... 16                                                                                                      
  23 360_guard    16                                                                                                        
  23 360_guard... 17                                                                                                      
  23 360_guard... 18              
  
  [work@xx:/data1/muce_temp/datamarts/reduce_result_file/302/1d/201212260000]$ LANG=En_US sort dd.dat     
  23 340_guard    16                                                                                                        
  23 340_guard    17                                                                                                        
  23 340_guard    18                                                                                                        
  23 360_guard    16                                                                                                        
  23 360_guard... 16                                                                                                      
  23 360_guard... 17                                                                                                      
  23 360_guard... 18                                 
  
  [work@xx:/data1/muce_temp/datamarts/reduce_result_file/302/1d/201212260000]$ LANG=en_US sort dd.dat     
  23 340_guard    16                                                                                                        
  23 340_guard    17                                                                                                        
  23 340_guard    18                                                                                                        
  23 360_guard... 16                                                                                                      
  23 360_guard    16          (why this line appear here ? )                                                                                      
  23 360_guard... 17                                                                                                      
  23 360_guard... 18      
  </code></pre>
  
  <p>the format details of rows in this file likes:</p>
  
  <pre><code>2^E3^F360_guard^E...^I16^Ee^E17/18^I63776769$
  2^E3^F360_guard^E^I16^Ee^E17/18^I63776769$
  2^E3^F360_guard^E...^I17^Ei^E0^I63776771$
  2^E3^F360_guard^E...^I18^Ei^E1^I63776773$
  </code></pre>
  
  <p>^E is '\x05' , ^F is '\x06', ^I is tab, $ is '\n' .</p>
  
  <p>Thanks in advance.</p>