Assertions include boundaries, which indicate the beginnings and endings of lines and words, and other patterns indicating in some way that a match is possible (including look-ahead, look-behind, and conditional expressions).
| Characters | 含义 |
|---|---|
^
|
Matches the beginning of input. If the multiline flag is set to true, also matches immediately after a line break character. For example,
/^A/
does not match the "A" in "an A", but does match the first "A" in "An A".
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,
/t$/
does not match the "t" in "eater", but does match it in "eat".
|
\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. 范例：
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,
/\Bon/
matches "on" in "at noon", and
/ye\B/
matches "ye" in "possibly yesterday".
|
注意
: The
?
character may also be used as a quantifier.
| Characters | 含义 |
|---|---|
x(?=y)
|
Lookahead assertion:
Matches "x" only if "x" is followed by "y". For example, /
Jack(?=Sprat)/
matches "Jack" only if it is followed by "Sprat".
/Jack(?=Sprat|Frost)/
matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results.
|
x(?!y)
|
Negative lookahead assertion:
Matches "x" only if "x"
is not followed by
"y"
.
例如，
/\d+(?!\.)/
matches a number only if it is not followed by a decimal point.
/\d+(?!\.)/.exec('3.141')
matches "141" but not "3.
|
(?<=y)x
|
Lookbehind assertion:
Matches "x" only if "x" is preceded by "y". For example,
/(?<=Jack)Sprat/
matches "Sprat" only if it is preceded by "Jack".
/(?<=Jack|Tom)Sprat/
matches "Sprat" only if it is preceded by "Jack" or "Tom". However, neither "Jack" nor "Tom" is part of the match results.
|
(?<!y)x
|
Negative lookbehind assertion:
Matches "x" only if "x" is not preceded by "y". For example,
/(?<!-)\d+/
matches a number only if it is not preceded by a minus sign.
/(?<!-)\d+/.exec('3')
matches "3".
/(?<!-)\d+/.exec('-3')
match is not found because the number is preceded by the minus sign.
|
// 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'.
使用
^
for matching at the beginning of input. In this example, we can get the fruits that start with 'A' by a
/^A/
regex. For selecting appropriate fruits we can use the
filter
method with an
arrow
函数。
let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"]; // Select fruits started with 'A' by /^A/ Regex. // Here '^' control symbol used only in one role: Matching beginning of an input. let fruitsStartsWithA = fruits.filter(fruit => /^A/.test(fruit)); console.log(fruitsStartsWithA); // [ 'Apple', 'Avocado' ]
In the second example
^
is used both for matching at the beginning of input and for creating negated or complemented character sets when used within
groups
.
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
例如，
/\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
断言
/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' ]
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'RegExp: Assertions' in that specification. |
For browser compatibility information, check out the main Regular Expressions compatibility table .