Regex by default tried to find the largest match possible (Greedy)
Common Tokens
. : Any character leaving newline
\w : Word Character ( a-z, A-Z, 0-9, _ )
\s : Space Characters (Space, Tabs, Newlines)
\u25b6 : Match an Unicode character
Anchors
They match invisible character that can occur before and after characters (^, $)
\b : Word (\w) Boundary
This character occurs between a “word character and a non word character” or “word character and start/ end of string”
Character Set
[] : Character Set (List of character to matched)
[1-7] : Match characters that fall between 1 and 7
^ : Inside character set is used to negate the characters
Quantifiers
* : 0 or More
+ : 1 or more
? : Above 2 followed by ? Lazy Match (Finds the smallest match)
? : 0 or 1
Groups
(r|s|rs) : Groups (Match different combinations & stores/ captures match)
(?:r|s|rs) : Non Capturing group
(?<name>) : Named Capture group
(\w)\w+\1 : Reference an capture group that is already declared
\k<name> : Backreference using name of Capture Group
Lookaround
(?=...) : Lookahead (They are not considered part of the capture)
(?!...) : Negated Lookahead
(?<=...) : Lookbehind
(?<!...) : Negated Lookbehind
Conditional Match
The ?(1) here refers to the first capture group
There is an alternative PCRE syntax for the same but it is not supported in python
(consition)?(?(1)true|false) : If else logic in regex