grep for complete word doesn't work when the word contains a dash

Question

I am greping for a whole word using -w flag and it isn't working correctly. I am assuming it's because the source words contain a "dash" ("-") character:

echo "sdf-a" > /tmp/grep.txt
~/Developer : cat /tmp/grep.txt 
sdf-a

should not return anything:

cat /tmp/grep.txt | grep -w a
sdf-a <<<<<<<<<<<<<<< BAD

I've also tried with the "\" option:

cat /tmp/grep.txt | grep  "\<a\>"
sdf-a << Should not return it

When I remove the dash from the source word:

~/Developer/ : echo "sdfa" > /tmp/grep.txt
~/Developer/ : cat /tmp/grep.txt | grep  "\<a\>" // nothing returned as expected
~/Developer/ : cat /tmp/grep.txt | grep -w a // nothing returned as expected

Answer 1

使用一个小红宝石例程（为了我的目的），它将与多个破折号匹配：

def grep(file,val)
    res = []
    text = File.open(file).read
    text.each_line { |l|
      if l.include?(val)
        res.push(l)
      end
    }
    if res.count == 0
      return nil
    end
    return res
  end