fix: fix parse regex bug
fix #1305
**Describe the bug**
I am trying to convert a time that I parsed using `dayjs.tz` into another timezone, but it appears to return the incorrect value.
This appears to be a regression, as it appears to work properly in `1.9.7`, though I'm not sure when this issue started.

In `1.9.7` (working properly): https://codepen.io/pikachews/pen/ZEpxwGE
![image](https://user-images.githubusercontent.com/5668416/103511419-e18b0500-4e1b-11eb-9574-991b0392750b.png)

In `1.10.1` (working incorrectly): https://codepen.io/pikachews/pen/NWRYoGo
![image](https://user-images.githubusercontent.com/5668416/103511385-cddf9e80-4e1b-11eb-99ee-a2f80bf7c26b.png)



**Expected behavior**
It should properly convert and return the correct time.

**Information**
 - Day.js Version: 1.10.1
 - OS: Windows 10
 - Browser: Chrome 87
 - Time zone: GMT-08:00 PST (Pacific Standard Time)]

Converting an timezoned instance to another timezone returns the wrong result
diff --git a/src/constant.js b/src/constant.js
index 875ef02b4..6be602e2c 100644
--- a/src/constant.js
+++ b/src/constant.js
@@ -26,5 +26,5 @@ export const FORMAT_DEFAULT = 'YYYY-MM-DDTHH:mm:ssZ'
 export const INVALID_DATE_STRING = 'Invalid Date'
 
 // regex
-export const REGEX_PARSE = /^(\d{4})[-/]?(\d{1,2})?[-/]?(\d{0,2})[^0-9]*(\d{1,2})?:?(\d{1,2})?:?(\d{1,2})?\.?(\d+)?$/
+export const REGEX_PARSE = /^(\d{4})[-/]?(\d{1,2})?[-/]?(\d{0,2})[^0-9]*(\d{1,2})?:?(\d{1,2})?:?(\d{1,2})?[.:]?(\d+)?$/
 export const REGEX_FORMAT = /\[([^\]]+)]|Y{1,4}|M{1,4}|D{1,2}|d{1,4}|H{1,2}|h{1,2}|a|A|m{1,2}|s{1,2}|Z{1,2}|SSS/g

diff --git a/test/parse.test.js b/test/parse.test.js
index 1941b7335..1dd38207e 100644
--- a/test/parse.test.js
+++ b/test/parse.test.js
@@ -174,6 +174,14 @@ describe('REGEX_PARSE', () => {
     expect(d).toBe(null)
   })
 
+  // format used in timezone plugin utcString
+  it('2021-1-4 0:42:53:000', () => {
+    const date = '2021-1-4 0:42:53:000'
+    const d = date.match(REGEX_PARSE)
+    expect(dayjs(date).valueOf()).toBe(moment(date).valueOf())
+    expect(d.join('-')).toBe('2021-1-4 0:42:53:000-2021-1-4-0-42-53-000')
+  })
+
   it('2020-12-31T18:00:00-05:00 (no regex match)', () => {
     const date = '2020-12-31T18:00:00-05:00'
     const d = date.match(REGEX_PARSE)

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