Regex

a - matches a
abc - matches abc
. - matches any character
[abc] - matches a,b or c
[^abc] - matches any character except newline

a* - zero or more a
a+ - one or more a
a? - zero or one a
a{3} - exactly 3 a's
a{2,} - two or more a's
a {2,5} - between 2 and 5 a's

(abc) groups abc together

^abc - matches abc only at the start of a line
abc$ - matches abc only at the end of the line
^$ - matches an empty line