Unix Regular Expression

Regular Expressions in grep, sed, awk

grep - global regular expression print

^ Caret match expression at the start of a line, as in ^A.
$ Question match expression at the end of a line, as in A$.
\ Back Slash turn off the special meaning of the next character, as in \^.
[ ] Brackets match any one of the enclosed characters, as in [aeiou]. Use Hyphen "-" for a range, as in [0-9].
[^ ]   match any one character except those enclosed in [ ], as in [^0-9].
. Period match a single character of any value, except end of line.
* Asterisk match zero or more of the preceding character or expression.
\{m,n\}   match m to n occurrences of the preceding.
\{m\}   match exactly m occurrences of the preceding.
\{m,\}   match m or more occurrences of the preceding.
\( \)   Remember this pattern for later usage.
\n   The n'th remembered pattern.

Use backreferences to find lines that contain two of the same lowercase letter in succession.

grep "\([a-z]\)\1" file

Enhanced Regular Expression Basic Syntax Reference