Regex_cheat_sheet

\d : [0-9] single     		\D : not of \d
\w : [0-9][a-z][A-Z] single    \W : not of \w
\s : whitespace or tab	\S : not of \s

Special Characters:
. —> any character (including spaces)
* —> 0 or more
+ —> 1 or more
x? —> x is character and which is optional
[ ] —> referring as set of characters appearing anyone from set
[a-zA-Z] —> any characters from a to z or A to Z
[^a-z] —> no characters from a to z (includes whitespace & newline)
( ) —> grouping characters
| —> or character used with grouping

Positioning characters:
^abc —> abc from beginning
abc$ —> abc from end
{a,b} —> length of words
or
{a} —> length of words
\b{a,b}\b—> word boundary matching len(a) to len(b)
    or
\b{a}\b

Note:
Use \. when refering the period (.)

Complex Examples:
rainbow.* —> rainbowcat , rainbowdog, rainbow… 
colou?r —> color, colour 
l[yi]nk —> link , lynk 
\d{3}[-.]\d{3}[-.]\d{4} —> 3 digits then - or . then 3 digit then - or . then 4 digit
			      —> 917-555-1234, 646.867-5309, 999-444.3333
[0-5] —> digits between 0 to 5
[^abc] —> not a but b or c —> bxxx , cxxx
(cats|dogs) —> matches exactly cats or dogs 

Advance Regex :
Obtain substring from string

Check out this for brushing skills
https://regexone.com/lesson/end?

Regex in Python
1) Obtaining whole matched string:
filtered_sheet_names = list(filter(expression.search, xls.sheet_names))
val = list(filter(exp.search,f.read().split("\n")))

2) Obtaining only particular matched part of string
re.findall("\d\d\d$",i)

3) Replacing matched string
re.sub("re1","string",i)



from re import match
values = ['123', '234', 'foobar']
filtered_values = list(filter(lambda v: re.match('^\d+$', v), values)) #Note: Complete match is necessary to obtain#

   
Example:    
Matching the ip address     
^[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]$