Assertions of regular series

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One of the components of an assertion is a boundary . For the text 、 Word or pattern , Boundaries can be used to indicate their starting or ending parts （ Such as forward assertion , Backward assertions and conditional expressions ）.

1、 Boundary class assertion

1.1、^

Match the beginning of the input . If the multiline mode is set to true,^ stay After newline It can also match immediately .

let reg = /^1[3-9]\d{9}/gm;
let str = "25000000000";
reg.test(str); // false
str = "15000000000\n17000000000";
let found = str.match(reg); // ['15000000000', '17000000000']

1.2、$

Match the end of the input . If the multiline mode is set to true,$ stay Before line break It can also match immediately .

let reg = /^1[3-9]\d{9}7$/gm;
let str = "25000000000";
reg.test(str); // false
str = "15000000000\n170000000007";
let found = str.match(reg); // ['170000000007']

1.3、\b

Match the boundary of a word （ Boundaries include line breaks and spaces ）, This is a word before the character / There is no character position after another word .

let reg = /\bm/gm;
let str = "month\nmoom";
str.match(reg); // ['m', 'm']
reg = /th\b/gm;
str = "month\nthird";
str.match(reg); // ['th']
str = "month\nsomething";
str.match(reg); // ['th']
str = "month ufo";
str.match(reg); // ['th']

1.4、\B

Match non word boundaries .

let reg = /on\B/gm;
let str = "at noon";
str.match(reg); // null
str = "noon noon\nnoon";
str.match(reg); // null
str = "ongoing";
str.match(reg); // ['on']

2、 Other assertions

2.1、x(?=y)

Forward assertion ： x By y Match when following x, Results do not include y.

let reg = /\d(?=y)/g;
let str = "1y2y3";
str.match(reg); // ['1', '2']

2.2、x(?!y)

Forward negation assertion ： x Has not been y Match when following x.

let reg = /\d(?!y)/g;
let str = "1y2y3";
str.match(reg); // ['3']

2.3、(?<=y)x

Assert backward ： x Follow y In the case of matching x, Results do not include y.

let reg = /(?<=y)\d/g;
let str = "y1y23";
str.match(reg); // ['1', '2']

2.4、(?<!y)x

A backward negative assertion ： x Do not follow y Match when x.

let reg = /(?<!y)\d/g;
let str = "y1y23";
str.match(reg); // ['3']

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