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Assertions에는 행이나 단어의 시작 · 끝을 나타내는 경계와 (앞, 뒤 읽고 조건식을 포함한) 어떤 식 으로든 매치가 가능한 것을 나타내는 다른 패턴이 포함됩니다.
| Characters | Meaning |
|---|---|
^ |
Matches the beginning of input. If the multiline flag is set to true, also matches immediately after a line break character. For example, This character has a different meaning when it appears at the start of a group. |
$ |
Matches the end of input. If the multiline flag is set to true, also matches immediately before a line break character. For example, |
\b |
Matches a word boundary. This is the position where a word character is not followed or preceded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero. Examples:
To match a backspace character ( |
\B |
Matches a non-word boundary. This is a position where the previous and next character are of the same type: Either both must be words, or both must be non-words, for example between two letters or between two spaces. The beginning and end of a string are considered non-words. Same as the matched word boundary, the matched non-word boundary is also not included in the match. For example, |
Note: The ? character may also be used as a quantifier.
| Characters | Meaning |
|---|---|
x(?=y) |
Lookahead assertion: Matches "x" only if "x" is followed by "y". For example, / |
x(?!y) |
Negative lookahead assertion: Matches "x" only if "x" is not followed by "y". For example, |
(?<=y)x |
Lookbehind assertion: Matches "x" only if "x" is preceded by "y". For example, |
(?<!y)x |
Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, |
// Using Regex boundaries to fix buggy string. buggyMultiline = `tey, ihe light-greon apple tangs on ihe greon traa`; // 1) Use ^ to fix the matching at the begining of the string, and right after newline. buggyMultiline = buggyMultiline.replace(/^t/gim,'h'); console.log(1, buggyMultiline); // fix 'tey', 'tangs' => 'hey', 'hangs'. Avoid 'traa'. // 2) Use $ to fix matching at the end of the text. buggyMultiline = buggyMultiline.replace(/aa$/gim,'ee.'); console.log(2, buggyMultiline); // fix 'traa' => 'tree'. // 3) Use \b to match characters right on border between a word and a space. buggyMultiline = buggyMultiline.replace(/\bi/gim,'t'); console.log(3, buggyMultiline); // fix 'ihe' but does not touch 'light'. // 4) Use \B to match characters inside borders of an entity. fixedMultiline = buggyMultiline.replace(/\Bo/gim,'e'); console.log(4, fixedMultiline); // fix 'greon' but does not touch 'on'.
입력 시작시 일치를 위해 ^를 사용하십시오. 이 예에서는 /^A/ regex로 'A'로 시작하는 결과를 얻습니다. 여기서 ^는 한 가지 역할 만합니다. 적절한 결과를 보기위해 화살표 함수가있는 필터 메소드를 사용합니다.
let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"]; // Select fruits started with 'A' by /^A/ Regex. // Here '^' control symbol used only in one role: Matching begining of an input. let fruitsStartsWithA = fruits.filter(fruit => /^A/.test(fruit)); console.log(fruitsStartsWithA); // [ 'Apple', 'Avocado' ]
두 번째 예제에서 ^는 두 가지 모두에 사용됩니다 : 입력의 일치 시작점, 그룹에서 사용될 때 부정 또는 보완 문자 세트.
let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"]; // Selecting fruits that dose not start by 'A' by a /^[^A]/ regex. // In this example, two meanings of '^' control symbol are represented: // 1) Matching begining of the input // 2) A negated or complemented character set: [^A] // That is, it matches anything that is not enclosed in the brackets. let fruitsStartsWithNotA = fruits.filter(fruit => /^[^A]/.test(fruit)); console.log(fruitsStartsWithNotA); // [ 'Watermelon', 'Orange', 'Strawberry' ]
let fruitsWithDescription = ["Red apple", "Orange orange", "Green Avocado"]; // Select descriptions that contains 'en' or 'ed' words endings: let enEdSelection = fruitsWithDescription.filter(descr => /(en|ed)\b/.test(descr)); console.log(enEdSelection); // [ 'Red apple', 'Green Avocado' ]
// JS Lookahead assertion x(?=y)
let regex = /First(?= test)/g;
console.log('First test'.match(regex)); // [ 'First' ]
console.log('First peach'.match(regex)); // null
console.log('This is a First test in a year.'.match(regex)); // [ 'First' ]
console.log('This is a First peach in a month.'.match(regex)); // null
For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches "141" but not "3.
console.log(/\d+(?!\.)/g.exec('3.141')); // [ '141', index: 2, input: '3.141' ]
Different meaning of ?! combination usage in Assertions /x(?!y)/ and Ranges [^?!].
let orangeNotLemon = "Do you want to have an orange? Yes, I do not want to have a lemon!"; // Different meaning of '?!' combination usage in Assertions /x(?!y)/ and Ranges /[^?!]/ let selectNotLemonRegex = /[^?!]+have(?! a lemon)[^?!]+[?!]/gi console.log(orangeNotLemon.match(selectNotLemonRegex)); // [ 'Do you want to have an orange?' ] let selectNotOrangeRegex = /[^?!]+have(?! an orange)[^?!]+[?!]/gi console.log(orangeNotLemon.match(selectNotOrangeRegex)); // [ ' Yes, I do not want to have a lemon!' ]
let oranges = ['ripe orange A ', 'green orange B', 'ripe orange C',]; let ripe_oranges = oranges.filter( fruit => fruit.match(/(?<=ripe )orange/)); console.log(ripe_oranges); // [ 'ripe orange A ', 'ripe orange C' ]
| Specification | Status | Comment |
|---|---|---|
| {{SpecName('ESDraft', '#sec-assertion', 'RegExp: Assertions')}} | {{Spec2('ESDraft')}} |
For browser compatibility information, check out the main Regular Expressions compatibility table.