Regular Expression

Basic Regular Expression

^A - Match with any line starts with A
A$ - Match with any line ends with A
A^ - Match with any line containging A^
$A - Match with any line containging $A
^^ - Match with any line containging ^^
$$ - Match with any line containging $$
[XYZ] - Match with any line containging X, Y or Z
[0-9] - Match wiht any line containging 0,1,...,9
[^A-Z] - Matches that doesn't contain A-Z.
. - Matches any character
* - Matches zero or more time
\{m\} - Matches if pattern repeats exactly m times.
\{m,n} - Matches if the pattern repeats m to n times.
\{,n} - Matches if the pattern repeats less or equal n times.
\{m,} - Matches if the pattern repeats more or equal to m times.
\<[tT]he\> - Matches exactly the/The.
$a$\1 - (PATTERN) remembers the pattern and the pattern can be used again by \n where n=[1,9]. So total nine patterns can be remembered and backreferenced. Another example: $[a-z]$$[a-z]$[a-z]\2\1 matches with any 5 letter palindrome.

echo -e "(123)-451-789\n(123)457-869" | grep "([0-9]\{3\})-[0-9]\{3\}-[0-9]\{3\}"