Regular Expression: Basic Regular Expression 
    Posix Regular Expression has three sets of compliance:BRE Basic Regular Expression, ERE: Extended Regular Expression and SRE: Simple Regular Expression. SRE is deprecated, in favour of BRE.
    In Posix Extended Regular Expression: There are 14 Metacharacter
    Basic Regular Expression DOES need to be escaped: [Preceded by a backslash \] Metacharacter
    Extended Regular Expression DOES NOT need to be escaped Metacharacter
Posix Regular Expression 14 metacharacters
[ ] Character Class
( ) Group
{ } Range
^ Begin of String
$ End of String
? Zero or One
. Any non-newline character
| Logic OR
+ One or more
- From To
POSIX Basic Regular Expression, Escape metacharacter ( ) and { }
POSIX Basic Regular Expression POSIX Extended Regular Expression
\ ( \ ) ( )
\{ \} { }
[ ]
\| \+ \? ^ $ ? . * | + -
Regular Expression: Logic OR 
    grep    'dog\|cat' f.txt        # POSIX Basic Regular Expression  BRE
                 ↑ 
                 + → BRE
    grep -E 'dog|cat' f.txt         # POSIX Extended Regular Regular  EBE
                ↑ 
                + →  ERE
Regular Expression: Range
Regular Expression: 3 to 5 digits 
    grep    'try[0-9]\{3,5\}.tex'   # POSIX Basic Regular Expression BRE
                     ↑     ↑ 
                     + - - + →  BRE
    grep -E 'try[0-9]{3,5}.tex'     # POSIX Extended Regular Expression ERE
                     ↑   ↑  
                     + - + →    ERE
Regular Expression: zero or one 
    grep    'try[0-9]\?.tex'    # POSIX Basic Regular Expression BRE
                     ↑ 
                     + → BRE
    grep -E 'try[0-9]?.tex'     # POSIX Extended Regular Expression ERE
                     ↑ 
                     + → ERE
POSIX Character Class 
    Character Class
    POSIX class  Similar to                  Meaning
    [:upper:]    [A-Z]                       uppercase letters
    [:lower:]    [a-z]                       lowercase letters
    [:alpha:]    [[:upper:][:lower:]]        upper- and lowercase letters
    [:alnum:]    [[:alpha:][:digit:]]        digits, upper- and lowercase letters
    [:digit:]    [0-9]                       digits
    [:xdigit:]   [0-9A-Fa-f]                 hexadecimal digits
    [:blank:]    [ \t]                       space and TAB characters only
    [:space:]    [ \t\n\r\f\v]               blank (whitespace) characters
    [:cntrl:]                                control characters
    [:graph:]    [^ \t\n\r\f\v]              printed characters
    [:print:]    [^\t\n\r\f\v]               printed characters and space