The square braces can also be used to specify what a single character cannot be, by use of the caret ˆ.

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The simplest kind of regular expression is a sequence of simple characters.
Due to the complexity and features of regular expressions, several signs are designed to better match characters while searching.

Basic Regular Expression Patterns

An on-going but a helpful book resource about NLP
https://web.stanford.edu/~jurafsky/slp3/

Speech and Language Processing (3rd ed. draft) 

The Kleene * means “zero or more occurrences of the immediately previous character or regular expression”.

Kleene *

The Kleene + means “one or more occurrences of the immediately preceding character or regular expression”.

Kleene +

This operator represents the "or" condition logic. Text strings at both sides of the operator represent conditions to be matched. The operator "|" is also called the pipe symbol.

E.g. /cat|dog/ -> matches either the string cat or the string dog.


Disjunction (OR) operator  | 

Enclosing patterns in parentheses makes the whole search pattern remains one string, while disjunction operator | can be placed within, to specify the variations of the search pattern.
E.g.  /gupp(y|ies)/ -> matches either the string guppy or guppies.


Parenthesis (grouping) operator ()

"*"  0 or more previous character(s) or expression(s)

"+" 1 or more previous character(s) or expression(s)

"? " 0 or 1 previous previous character(s) or expression(s)

{n}/ {n,}/ {,m}/ {n,m} exact number or number range of occurrence

Operators for counting (regular expression)

\d one digit from 0 to 9

\D one character that is not a digit

\w one word character

\W one character that is not a word character

\s one whitespace character

\S one character that is not a whitespace character 

Aliases for common ranges in regular expression

The string of characters inside the braces specifies a disjunction of characters to match.

Disjunction: []

Dash (-) can be used to specify any one character in a range.

Range: -

Anchors are special characters that anchor regular expressions to particular places in a string.

Anchors: $

Exclude: ^

The question mark /?/ means “the preceding character or nothing”.

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