正規表現における境界線「閾値」と「閾値」を徹底解説

// Only the first and last positions match
console.log('0aZ_'.replace(/\b/g, '.')) // .0aZ_.

// + is not \w, so its left and right gaps can be matched
console.log('a+a'.replace(/\b/g, '.')) // .a.+.a.

// The space is not \w either, so its left and right gaps can be matched
console.log('a a'.replace(/\b/g, '.')) // .a. .a.

console.log('0aZ_'.replace(/\B/g, '.')) // 0.a.Z._

console.log('a+a'.replace(/\B/g, '.')) // a+a.

console.log('a a'.replace(/\B/g, '.')) // a a

var str = ' 2 ';//where the position between the space and the 2 is called the word boundary, matching \b

var str = ",,,,, and huh ,,,,,";
var reg = '\B huh \B';//reg matches the middle huh, which is flanked by characters, with the middle position being a non-word boundary.

var test = "137,1,33,4,3,6,21,3,35,93,2,98"; 
var count = test.match(test, "\b3\b").length; //result: 2

'7654321'.replace(/\B(? =(\d{3})+(?! \d))/g,',')
//7,654,321 (matches non-word boundaries with non-numeric endings and consecutive 3-digit integer multiples in the middle)

'99893'.replace(/\B(? =(\d{3})+$)/g, ',')
// '99,893'

'12345678901'.replace(/\B(? =(? :\d{4})+$)/g, '-')
// '123-4567-8901'