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Regular Expressions in Linux

Regular Expression:

[.] DOT dot matches for a single character
[abcd] matches one of the list ,say [abcd] matches one of a,b,c,d
[a-d] matches the range of characters a to d
[^a-i] should not contain the range a to i
[a*] ~= {0,} match zero or more a’s
[a+] ~= {1,} match one or more a’s
[a?] ~= {0,1} zero or one occurance of letter a
wh(at|en|ere} what when where , you can specify group of letters and also a pipe to mark OR operator
^not start with string not,notify,notice etc……
not$ end with not like knot

 

GREP: search Globally for Regular Expression and Print

grep option[s]
  -n number in the file
  -c count the number of occurrences, unlike wc –l it gives the file wise count
  -A context selection After
-B Context selection Before
you can specify number with A or B
  -l List the names of files
  -r recursively Grep
  -s , if there are permission errors it suppresses
  -e used to search multiple regular expressions

Egrep: Extended GREP { More operators +,?,(),|}

Fgrep: Fixed length Grep , does not support operators

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