速记

[0-9] The square brackets are not literally matched because they are treated specially as metacharacters.
character escape would lead to misleading, try to avoid it: not always supported and less flexibility of character class. Use digit character class.
\D = [^0-9] = [^\d]
\w = [0-9a-zA-Z], \W = [^\w] 

enclose regex in a pair of parenthesis to place it in a group, and use \1 to back reference it. like (\d)0\1 -> 707 or g/(word1)(word2)/\2 \1/
enclose digit in a pair of curly brace to describe last regex's occurrences.
enclose a set of characters (or character class) by `[]`
some quantifiers: . * ? + 
alternation: |
^$
dotall means a dot will match all characters, including newlines.
^.* with dotall mode will match whole text.

word boundaries: \bword\b, \<word\> (older syntax, may not be avail in recent applications), \Aword\Z (Perl and PCRE)
Quoting a Group of Characters as Literals: \Q$\E, or \$   (.^$*+?|(){}[]\-) 
(?i) case insensitive; (?m) multiple line;  (?s) single line / dotline;  (?x) ignore whitespace  

??? (?<named_group>pattern) and  $+{named_group} to back reference; use (?<name>pattern)aaaaaa<name> to reuse. ###### by language……..
(?:the|The|THE)   non-capturing groups: do not need to store in memory.
(?>the)  to turn off backtracking.

Greedy (try to match whole string), Lazy, and Possessive ( one attempt)  
 ? Zero or one (optional) 
+ One or more
* Zero or more
{n} Match n times exactly 
{n,} Match n or more times 
{m,n} Match m to n times
{0,1} Same as ? (zero or one) 
{1,0} Same as + (one or more) 
{0,} Same as * (zero or more)
?? Lazy zero or one (optional) 
+? Lazy one or more
*? Lazy zero or more
{n}? Lazy n
{n,}? Lazy n or more 
{m,n}? Lazy m,n

Positive Lookaheads: time (?=results) #=> find out 'time' in 'time result' (((((( try it in ack/ag
or negative lookaheads:  (?!results)
positive lookbehinds:   (?<=time) result
Negative Lookbehinds: (?1)(?<!ancyent) marinere 

% ack -i 'return (?=os)' ark_wsgi.py
            return os.popen(filename[1:], 'w', 1)
% ack -i '(?<=os).popen' ark_wsgi.py
            return os.popen(filename[1:], 'w', 1)

matching html start tags: <[_a-zA-Z][^>]*>

Glossary 
pesky  wildcard   parenthesis - parentheses  caret  decipher
Is your head spinning?  gobble  nibble   archaic