JavaScript Regular Expressions

This matches one or more occurrences and is equivalent to {1,}.
/o+/ matches "oo" in "foo".
?
This matches zero or one occurrences and is equivalent to {0,1}.
/fo?/ matches "fo" in "foo" and matches "f" in "fairy".
+?
*?
"?" can also be used following one of the *, +, ?, or {} quantifiers to make the later match nongreedy, or the minimum number of times versus the default maximum.
/\d{2,4}?/ matches "12" in the "12345" string, instead of "1234" due to "?" at the end of the quantifier nongreedy.
x(?=y)
Positive lookahead: It matches x only if it's followed by y. Note that y is not included as part of the match, acting only as a required condition.
/Java(?=Script|Hut)/ matches "Java" in "JavaScript" or "JavaHut", but not "JavaLand".
x(?!y)
Negative lookahead: It matches x only if it's not followed by y. Note that y is not included as part of the match, acting only as a required condition.
/^\d+(?! years)/ matches "5" in "5 days" or "5 books", but not "5 years".

Quantifiers

In the following table, you can find the patterns for quantifiers, which specify how many instances of a character, group, or character class must be present in an input for a match to be found.
Pattern
Description
Example
{n}
This matches exactly n occurrences of a regular expression.
/\d{5}/ matches "12345" (five digits) in "1234567890".
{n,}
This matches n or more occurrences of a regular expression.
/\d{5,}/ matches "1234567890" (minimum of five digits) in "1234567890".

Character sets

In the following table, you can find the patterns for character sets, which tell the Regex to match only one character out of several characters.
Pattern
Description
Example
[xyz]
This matches any one character enclosed in the character set. You can use a hyphen to denote the range. For example, /[a-z]/ matches any letter in the alphabet and matches /[0-9]/ to any single digit.
/[ao]/ matches "a" in "bar"
[^xyz]
This matches any one character, which is not enclosed in the character set.
/[^ao]/ matches "b" in "bar"