JavaScript Regex Cheat Sheet

Successfully working with regular expressions requires you to know what each special character, flag, and method does. This is a regular expressions cheat sheet which you can refer to when trying to remember how a method, special character, or flag works.

Creating a Regular Expression in JavaScript

Matching a Specific Set of Characters

Specifying the Number of Times to Match

Using Flags With Regular Expressions

Parenthesis in Regular Expressions

Regular Expression Methods in JavaScript

Creating a Regular Expression in JavaScript

There are two ways of defining a regular expression in JavaScript.

Regular Expression Literal

var rgx = /^(\d+)/: You can use a regular expression literal and enclose the pattern between slashes. This is evaluated at compile time and provides better performance if the regular expression stays constant.

Constructor Function

var rgx = new RegExp('^(\d+)'): The constructor function is useful when the regular expression may change programmatically. These are compiled during runtime.

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Matching a Specific Set of Characters

The following sequences can be used to match a specific set of characters.

Any Character .

The period matches any character except the newline '\n'.

Custom Character Groups [...]

The square brackets are used to specify a collection of characters. [abc] would match the letter a, b, or c. [a-z] would match any letter between a and z. 

With a ^ symbol, the square brackets match any character not listed. For example, [^0-9] matches any non-digit character.

Word Characters \w and \W

\w matches word characters. Word characters are alphanumeric (a-z, A-Z characters, and underscore). \W matches non-word characters: everything except alphanumeric characters and underscore.

Digit Characters \d and \D

\d matches digit characters—any digit from 0 to 9. It's equivalent to [0-9] in bracket notation. \D matches non-digit characters, i.e. everything except 0 to 9. It's equivalent to [^0-9] in bracket notation.

Whitespace Characters \s and \S

\s matches whitespace characters. This includes spaces, tabs, and line breaks. Whitespace is equivalent to [\t\n\r\f\v] with bracket notation. \S matches all other characters except whitespace. It's equivalent to [^ \t\n\r\f\v].

Beginning or End of Word \b or Not \B

\b matches the beginning or end of a word.

\B matches only within a word.

Caret ^

The caret symbol ^ matches the beginning of the string.

Dollar $

The dollar symbol $ matches the end of the string.

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Specifying the Number of Times to Match

All the meta-characters given below can be used to evaluate patterns based on the number of times a character is repeated. You can add quantifiers to specify how many characters should be included in the match at once.

Star *

The * symbol is used to check if there are zero or more occurrences of the pattern to the left of the symbol.

Plus +

The + symbol is used to check if there is at least one occurrence of the pattern to the left of the symbol.

Question ?

The ? symbol is used to check if there is zero or one occurrence of the pattern to the left of the symbol.

Number of Repetitions {n} or {n,m}

Braces with a single number denote a specific number of repetitions. If the brace has two numbers, in the format {n,m}, n denotes the minimum, and m denotes the maximum number of repetitions of the pattern to the left.

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Using Flags With Regular Expressions

Flags can be used to control how a regular expression should be interpreted. You can use flags either alone or together in any order you want. These are the five flags which are available in JavaScript. 

Global g

g: search the string for all matches of a given expression instead of returning just the first one.

Case-Insensitive i

i: make the search case-insensitive so that words like apple, Apple, and APPLE can all be matched at once.

Multiline Mode m

m: make the ^ and $ tokens look for a match at the beginning or end of each line instead of the whole string.

Unicode Mode u

u: enable Unicode code point escapes in your regular expression. Not all browsers support this option. 

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var regex = new regexp('\u{61}', 'u');

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console.log(regex.unicode); // true

Sticky Search y

y: only look for a match at the current position in the target string. lastindex indicates where the search will begin.

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Parenthesis in Regular Expressions

The parenthesis is used for grouping. Sub patterns can be formed using ().

You can also combine the or | operator with () to combine pick from a number of possible patterns.

Finally, the parts of a matching string that match some pattern in brackets will be broken out and returned separately. This means you can use regular expressions to extract or modify data from a string. 

It's with nested patterns that the power of regular expressions really starts to become apparent.

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Regular Expression Methods in JavaScript

The regular expressions that you create using the flags and character sequences we have discussed so far are meant to be used with JavaScript methods to search, replace, or split a string. Here are some methods related to regular expressions.

Test If a Pattern Matches test()

test(): check if the main string contains a substring which matches the pattern specified by the given regular expression. It returns true on a successful match and false otherwise.

search(): check if the main string contains a substring which matches the pattern specified by the given regular expression. It returns the index of the match on success and -1 otherwise.

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var texta = 'i like apples very much';

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var regexone = /apples/;

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var regextwo = /apples/i;

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// output : -1

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console.log(texta.search(regexone));

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// output : 7

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console.log(texta.search(regextwo));

Get Matches match() and exec()

match(): search if the main string contains a substring which matches the pattern specified by the given regular expression. If the g flag is enabled, multiple matches will be returned as an array.

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var texta = 'all I see here are apples, Apples and APPLES';

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var regexone = /apples/gi;

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// output : [ "apples", "Apples", "APPLES" ]

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console.log(texta.match(regexone));

exec(): search if the main string contains a substring which matches the pattern specified by the given regular expression. The returned array will contain information about the match and capturing groups.

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var texta = 'do you like apples?';

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var regexone = /apples/;

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// output : apples

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console.log(regexone.exec(texta)[0]);

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// output : do you like apples?

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console.log(regexone.exec(texta).input);

Replace Matched Text replace()

replace(): search for a substring matching the given pattern and replace it with the provided replacement string.

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var texta = 'Do you like apples?';

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var regexone = /apples/i

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// output : Do you like mangoes?

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console.log(texta.replace(regexone, 'mangoes'));

Split on Matches split()

split(): this method will allow you to split the main string into substrings based on the separator specified as a regular expression.

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var texta = 'This 593 string will be brok294en at places where d1gits are.';

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var regexone = /\d+/g

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// output : [ "This ", " string will be brok", "en at places where d", "gits are." ]

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console.log(texta.split(regexone))

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Conclusion

In previous tutorials, we have covered the basics of regular expressions as well as some more complicated expressions which can prove to be useful every once in a while. These two tutorials explained how different characters or character sequences work in regular expressions.