Regex

Regex cheatsheet.

Links

Regex is a way to search through the text to validate text, find and replace, etc.

Format

Start and end with a /. Everything within the forward slash is the regex:

/cat/g

Expression flags

The g in /cat/g is an expression flag. An expression flag defines the scope of the regex search. The expression flags are listed below:

g: global
i: case insensitive
m: multiline
s: single line
u: unicode
y: sticky

Generally, you will use the g flag the most.

By default, a regex pattern returns only the first result that it finds. To return all, use the /g expression flag.

Character sets

[bcf]at matches the following:

Ranges

Ranges match any single character, digit, or symbol within the range. [a-z]at matches any word that begins with a lowercase letter and ends with at.

Range type	Example
Partial	[a-f] or [g-p]
Capitalized	[A-Z]
Digit	[0-9]
Symbol	[#$%&@]
Mixed	[a-zA-Z0-9]

Repeating characters

Specify the number of repeating characters with the {val} syntax:

Expression	Match
`a{5}`	`aaaaa`
`[a-z]{4}`	any four-letter lowercase word
`[a-z]{6,}`	any lowercase word with 6 or more letters
`[a-z]{8,11}`	any lowercase word with between 8 or 11 letters (inclusive)
`[0-9]{11}`	11-digit number

Metacharacters

Write compact regex with metacharacters:

Expression	Match
`\d`	any digit, equal to `[0-9]`
`\w`	any word character, such as a letter or digit.
`\W`	any character that is not a word character or digit.
`\s`	any whitespace
`\S`	anything other than a whitespace character.
`\t`	tab character

Combine these with repeating characters:

\w{5}: five-letter word or number
\d{11}

Special characters

Character	Description	Example
`+`	One or more of the previous characters.	`c+at` matches `cat` or `ccccat`
`?`	Zero or one of the previous characters.	`c?at` matches `cat` or `at`
`*`	Zero or more of the previous characters.	`c*at` matches `at` or `cat` or `cccat`
`\`	Escape character.	`\d` matches `d`
`[^]`	Negate notation. Do not match the characters after the `^` within the braces.	`b[^a]ld` matches `bold` but not `bald`
`.`	Match any digit, letter, or symbol except newline.	`.{8}` matches any eight-character token.

For example:

.+ matches one or more unlimited number of characters.
[a-z]+ matches all lowercase words.

Groups

Groups apply pattern matching to a section of the expression. book(ing)? matches book and booking.

Alternate characters

Use the pipe (|) character to match one or the other option. For example:

bat|bit

You can match the same values by enclosing the options in parentheses:

b(a|i)t

Starting and ending characters

REVISIT THIS

Character	Description	Example
`^`	Matches pattern at the start of the string. Place this at the beginning of the pattern.
`$`	Matches pattern at the end of the string. Place this at the end of the pattern.

Quantifiers

Quantifiers define how many

+: match one or more of the preceding token.
?: optional. Optionally, you want to match the preceding token.
*: match zero or more of the preceding token. Wildcard.
.: match anything except a newline.
\.: search for a period.
{min,max}: match any characters between min and max. So /\w{4,5}\/g matches any 4 or 5 consequtive word characters.
[bc]at: matches bat or cat.
[a-zA-Z]: any word that ends in at and starts with an uppercase or lowercase letter. Works with numbers too ([0-9])
(): groups

Lookaheads

Read this Sitepoint article.