需要帮助为用户名编写正则表达式

Question

Hey so I'm looking for a regrex pattern that:

• Allows for only two or one _ in a row but no more, can end start and have the _ in between
• Max 20, min 3 characters
• Allows for only one period in a row and can't start or end in a period
• Accepts AZ and 1-9

Some examples of passes and fails are:

.test #fail
_test #pass
_.test #pass
__test #pass
test_ #pass
te_.st #pass
te.st #pass
..test #fail

So far I have ^[\\w](?!.*?\\.{2})(?!.*?_{2})[\\w.]{1,28}$ , which works for everything but detecting the __ between and at the end of the words. Any help would be appreciated! This has been hurting my brain for hours

Answer 1

Allows for only one period in a row and can't start or end in a period

Negative lookahead for ^\\. , \\.$ , and \\.\\. from the beginning of the string

Allows for only two or one _ in a row but no more, can end start and have the _ in between

Negative lookahead for __ from the beginning of the string

Max 20, min 3 characters

After the other logic is implemented, match and consume 3-20 of the allowed characters

^(?!.*___|\.|.*\.$|.*\.\.)[a-z1-9_.]{3,20}$

https://regex101.com/r/TUjY13/2

• ^ - From start of string:
• (?!.*___|\\.|.*\\.$|.*\\.\\.) - Make sure match doesn't:
  • .*___ - contain triple underscores
  • \\. - start with a .
  • .*\\.$ - end with a .
  • .*\\.\\. - contain double periods
• [a-z1-9_.]{3,20} - Match 3-20 characters in the string body using all allowed characters (case-insensitive)