VAR='[[:alpha:]]'

[[ $VAR =~ '[[:alpha:]]' ]] && echo match 1

[[ a =~ '[[:alpha:]]' ]] || echo match 2

[[ a =~ [[:alpha:]] ]] && echo match 3

[[ a =~ $VAR ]] && echo match 4

[[ a =~ "$VAR" ]] || echo match 5

line=aab
[[ $line =~ [[:space:]]*(a)?b ]] && echo match 6

V="alphabet"
[[ $V == alphabet ]] && echo yes 1
[[ $V == "alphabet" ]] && echo yes 2
[[ $V == 'alphabet' ]] && echo yes 3
[[ $V =~ alphabet ]] && echo yes 4
[[ $V =~ "alphabet" ]] && echo yes 5
[[ $V =~ 'alphabet' ]] && echo yes 6

DOG="Dog name - 01 - Wiggles"
REPAT='([[:alpha:][:blank:]]*)- ([[:digit:]]*) - (.*)$'
if [[ $DOG =~ ([[:alpha:][:blank:]]*)-\ ([[:digit:]]*)\ -\ (.*)$ ]]
then
   echo Dog ${BASH_REMATCH[2]} is ${BASH_REMATCH[3]}
fi
if [[ $DOG =~ $REPAT ]]
then
   echo Dog ${BASH_REMATCH[2]} is ${BASH_REMATCH[3]}
fi

[[ $REPAT =~ "$REPAT" ]] && echo rematch 1

v="one two buckle my shoe"
[[ ${v} =~ "one two" ]] && echo matches 7

[[ ${v} =~ (one two) ]] && echo matches 8

[[ ${v} =~ one\ two ]] && echo matches 9

unset pattern string
pattern="xxx.yyy"
string=xxxAyyy

[[ $string =~ $pattern ]] && echo unquoted matches
[[ $string =~ "$pattern" ]] && echo quoted matches

# problems in pre-patched bash-4.2
[[ "hello
world" =~ llo
 ]] && echo match control-a 1
[[ "hello
world" =~ 
world ]] && echo match control-a 2
[[ "hello
world" =~ 
world$ ]] && echo match control-a 3
[[ "hello
world" =~ 
 ]] && echo match control-a 4
[[ "hello
world" =~ o
world$ ]] && echo match control-a 5