Regular Expressions

Content: Regular Expressions

Metacharacters

Metacharacter	Description
.	Matches any single character except newline and line terminator
\d	Matches any digit
\D	Matches any non-digit
\s	Matches any whitespace character
\S	Matches any non-whitespace character
\w	Matches any word character (alphanumeric plus underscore)
\W	Matches any non-word character
\b	Matches a word boundary
\B	Matches a non-word boundary

Quantifiers

Quantifier	Description
+	Matches 1 or more occurrences
*	Matches 0 or more occurrences
?	Matches 0 or 1 occurrence, can denote laziness to prevent greedy matches
{n}	Matches exactly n occurrences
{n,m}	Matches between n and m occurrences
{n,}	Matches at least n occurrences
^n	Matches any string with n at the beginning
[^abc]	When inside square brackets, denotes negation
n$	Matches any string with n at the end

Modifiers

Modifier	Description
i	Case insensitive matching (/n/i matches n regardless of case)
g	Global search (/n/g searches for all occurrences of n)
m	Multiline matching

Square Brackets

Expression	Description
[abc]	Matches any character inside the brackets
[^abc]	Matches any character not inside the brackets
[0-9]	Matches any digit from 0 to 9
[a-z]	Matches any lowercase letter from a to z
[A-Z]	Matches any uppercase letter from A to Z
[A-z]	Matches any letter from A to z
[red|blue|green]	Matches any of the specified alternatives

Ways to Create a Regular Expression

There are two ways to create a regular expression: using a literal or using a constructor.

var reg = new RegExp("\\d");
var reg = /\d/;

Regular Expression API

1. RegExp Methods

Method	Description
test	Searches for a specified value in a string, returns true or false
exec	Searches for a specified value in a string, returns the found value and its position

2. String Methods

Method	Description
search	Searches for a match against a regular expression
match	Finds one or more matches of a regular expression
replace	Replaces matches with a regular expression
split	Splits a string into an array by matching against a regular expression

Additional Notes:

• Regular expressions are powerful tools for text processing and can greatly simplify tasks such as validation, parsing, and string manipulation.
• When using regular expressions, especially with user input, it’s important to consider performance and security implications such as catastrophic backtracking and injection attacks.
• It’s a good practice to comment complex regular expressions to explain their purpose and pattern, making maintenance easier for others or for future review.
• Tools like regex testers and visualizers can be immensely helpful when creating or debugging regular expressions.