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,
|
x+ |
Matches the preceding item "x" 1 or more times. Equivalent to
|
x? |
Matches the preceding item "x" 0 or 1 times. For example,
If used immediately after any of the quantifiers |
x{n} |
Where "n" is a positive integer, matches exactly "n" occurrences of
the preceding item "x". For example, |
x{n,} |
Where "n" is a positive integer, matches at least "n" occurrences of
the preceding item "x". For example, |
x{n,m} |
Where "n" is 0 or a positive integer, "m" is a positive integer, and
|
|
By default quantifiers like
|
Examples
Repeated pattern
js
const wordEndingWithAs = /\w+a+\b/;
const delicateMessage = "This is Spartaaaaaaa";
console.table(delicateMessage.match(wordEndingWithAs)); // [ "Spartaaaaaaa" ]
Counting characters
js
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
js
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
js
const text = "I must be getting somewhere near the center of the earth.";
const greedyRegexp = /[\w ]+/;
// [\w ] a letter of the latin alphabet or a whitespace
// + one or several times
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