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Boundaries Indicate the beginnings and endings of lines and words.
Types
| Characters | Meaning |
|---|---|
^ |
Matches 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 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]), see Character Classes. |
\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. Such as 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, |
Examples
General overview 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'.
Matching beging of an input by a ^ control character.
Use ^ for average matching begining of a word. In this example get fruites that starts with 'A' by a /^A/ regex. Here ^ plays only one role: show begining of the input. For selecting appropriate fruits used filter method with an arrow function.
let fruits = ["Apple", "Watermelon", "Orange", "Avocado", "Strawberry"]; // Select fruits started with 'A' by /^A/ Regex. // Here '^' control symbol used only in one role: Mathcing begining of an input. let fruitsStartsWithA = fruits.filter(fruit => /^A/.test(fruit)); console.log(fruitsStartsWithA); // [ 'Apple', 'Avocado' ]
It the second example ^ used for both: matching begining of the input and as negated or complemented character set when it used in 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' ]
Matching a word boundary.
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' ]