Regular Expression is a special text string for describing a search pattern
Letters: abc
Digits: 123
Not Expression: ^
In some card games, the Joker is a wildcard and can represent any card in the deck.
Similarly, there is the concept of a wildcard, which is represented by the . (dot) metacharacter, and can match any single character (letter, digit, whitespace, everything).
.
E.g:
... can match abc, 234, 3rZ
To escape a dot, (pattern match .), use backlash \.
[abc] : Only a or b or c
[^abc]: Not a b, nor c
\d : digit
\D : not digit
\w : word
\W : not word
\s : space
\S : not space
https://www.regular-expressions.info/refshorthand.html
Number of occurance of character is specified with {} sign
{4}: repeat 4 times
{2,4}: repeat from 2 to 4 times
E.g:
a{3,5}
*: Zero or more repetitions
+: at least one repetitions
We could use the above regular expression with a replacement pattern - $3$2$1$2$4 or \3\2\1\2\4.
RegexOne - Learn Regular Expression with simple, interactive exercies
Regxr - Live, responsive tool to learn, build and test regex
Regex: Quantifiers
Regex: Capturing Groups and Backreferences
Regex: Character Classes
Regex: Shorthand Character Classes
Some simple pattern