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Quantifiers

Quantifiers indicate numbers of characters or expressions to match.

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Types​

[!NOTE] In the following, item refers not only to singular characters, but also includes character classes and groups and backreferences.

Characters Meaning 
x*

Matches the preceding item "x" 0 or more times. For example, /bo*/ matches "boooo" in "A ghost booooed" and "b" in "A bird warbled", but nothing in "A goat grunted". 

x+

Matches the preceding item "x" 1 or more times. Equivalent to {1,}. For example, /a+/ matches the "a" in "candy" and all the "a"'s in "caaaaaaandy". 

x?

Matches the preceding item "x" 0 or 1 times. For example, /e?le?/ matches the "el" in "angel" and the "le" in "angle."

If used immediately after any of the quantifiers *, +, ?, or {}, makes the quantifier non-greedy (matching the minimum number of times), as opposed to the default, which is greedy (matching the maximum number of times). 

x{n}

Where "n" is a non-negative integer, matches exactly "n" occurrences of the preceding item "x". For example, /a{2}/ doesn't match the "a" in "candy", but it matches all of the "a"'s in "caandy", and the first two "a"'s in "caaandy". 

x{n,}

Where "n" is a non-negative integer, matches at least "n" occurrences of the preceding item "x". For example, /a{2,}/ doesn't match the "a" in "candy", but matches all of the a's in "caandy" and in "caaaaaaandy". 

x{n,m}

Where "n" and "m" are non-negative integers and m >= n, matches at least "n" and at most "m" occurrences of the preceding item "x". For example, /a{1,3}/ matches nothing in "cndy", the "a" in "candy", the two "a"'s in "caandy", and the first three "a"'s in "caaaaaaandy". Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more "a"s in it. 

x*?

x+?

x??

x{n}?

x{n,}?

x{n,m}?

By default quantifiers like * and + are "greedy", meaning that they try to match as much of the string as possible. The ? character after the quantifier makes the quantifier "non-greedy": meaning that it will stop as soon as it finds a match. For example, given a string like "some <foo> <bar> new </bar> </foo> thing":

  • /<.*>/ will match "<foo> <bar> new </bar> </foo>"
  • /<.*?>/ will match "<foo>"

Examples​

Repeated pattern​

In this example, we match one or more word characters with \w+, then one or more characters "a" with a+, and finally end at a word boundary with \b.

const wordEndingWithAs = /\w+a+\b/;
const delicateMessage = "This is Spartaaaaaaa";

console.table(delicateMessage.match(wordEndingWithAs)); // [ "Spartaaaaaaa" ]

Counting characters​

In this example, we match words that have a single letter, words that have between 2 and 6 letters, and words that have 13 or more letters.

const singleLetterWord = /\b\w\b/g;
const notSoLongWord = /\b\w{2,6}\b/g;
const longWord = /\b\w{13,}\b/g;

const sentence = "Why do I have to learn multiplication table?";

console.table(sentence.match(singleLetterWord)); // ["I"]
console.table(sentence.match(notSoLongWord)); // [ "Why", "do", "have", "to", "learn", "table" ]
console.table(sentence.match(longWord)); // ["multiplication"]

Optional character​

In this example, we match words that either end with "our" or "or".

const britishText = "He asked his neighbour a favour.";
const americanText = "He asked his neighbor a favor.";

const regexpEnding = /\w+ou?r/g;
// \w+ One or several letters
// o   followed by an "o",
// u?  optionally followed by a "u"
// r   followed by an "r"

console.table(britishText.match(regexpEnding));
// ["neighbour", "favour"]

console.table(americanText.match(regexpEnding));
// ["neighbor", "favor"]

Greedy versus non-greedy​

In this example, we match one or more word characters or spaces with [\w ]+ and [\w ]+?. The first one is greedy and the second one is non-greedy. Note how the second one stops as soon as it meets the minimal requirement.

const text = "I must be getting somewhere near the center of the earth.";
const greedyRegexp = /[\w ]+/;

console.log(text.match(greedyRegexp)[0]);
// "I must be getting somewhere near the center of the earth"
// almost all of the text matches (leaves out the dot character)

const nonGreedyRegexp = /[\w ]+?/; // Notice the question mark
console.log(text.match(nonGreedyRegexp));
// "I"
// The match is the smallest one possible

See also​