Packet
/(\d{4})-(\d{2})-(\d{2})/.test('2019-08-25 ') // to true
the RegExp. $. 1 =' 2019 '
the RegExp. $ 2 '08 = '
the RegExp. $ . 3 = '25'
the RegExp. $ . 4 = ''
the RegExp. $ _ = '2019-08-05' // up to RegExp. $ 9, $ 10 is no, which in itself is js pit @ solution:
'2019-08-25'.match (/ (\ {D}. 4) - (\ D {2}) - (\ D {2}) / );
// 0: "2019-08-25"
// . 1: "2019"
// 2: "08"
// . 3: "25"
// Groups: undefined
// index: 0
// INPUT: "2019-08-25"
// length:. 4
ES2018 extension of regular (packet):
'2019-08-25'.match(/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/)
// 0: "2019-08-25"
// 1: "2019"
// 2: "08"
// 3: "25"
// groups: {year: "2019", month: "08", day: "25"}
// index: 0
// input: "2019-08-25"
// length: 4
Quote
'2019-08-25'.match (/ (\ {D}. 4) - (\ D {2}) - \ 2 / )
// null
' 2019-08-08'.match (/ (\. 4 {D }) - (\ D {2}) - \ 2 / )
// not null
// last '\ 2' is a reference to the second
// ES2018 reference
'2019-08-25'.match (/ (<year> \. 4 {D}?) - (? <month The> \ D {2}) - \ K <month The> /) // null
'2019-08-08'.match (/ (<year>? \. 4 {D}) -? (<month The> \ D {2}) - \ K <month The> /) // not null
'2019-08-25'.replace (/ (\ {D}. 4) - (\ D {2}) - (\ D {2}) /, `year ($. 1), month The ($ 2 )`)
// "year (2019), month the (08)"
// $. 1, $ 2 is a reference to the first two strings match