Is my regex correct? ^(?!_|[0-9])(?!.*__)[A-Za-z0-9_]+(?<!_)$

Is my regex correct? ^(?!_|[0-9])(?!.*__)[A-Za-z0-9_]+(?<!_)$

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This is what I came up with for the 4 requirements you listed:


I am not sure what you are doing with your regex, but to me it does not look valid. If you want to test it out for yourself, try, 38 regular expressions found in this category! __ | | a--b@c__d.ef ^(([A-Za-z0-9]+_+)|([A-Za-z0-9]+\-+)|([A-Za-z0-9]+\. Some unconventional, yet technically valid, addresses will not be matched, but this is only a� Regex to repeat the character [A-Za-z0-9] 0 or 5 times needed. i do have regex expression that i can try between a range [A-Za-z0-9] {0,5}. But i dont want it to operate in the range, i want it to be for fixed number of times (either 0 or 5).

Your pattern fulfills your requirements. Instead of using negative lookbehind for a _, match [A-Za-z0-9] at the end (the character set without a _):


You can also simplify [A-Za-z0-9_] to \w, [0-9] to \d, use the case-insensitive flag instead of repeating a-zA-Z, and combine the initial negative lookaheads if you wish:


Regular Expression (Regex) Tutorial, E.g., the regex x matches substring "x" ; regex 9 matches "9" ; regex = matches "=" ; and regex @ matches "@" For ASCII, word characters are [a-zA-Z0-9_]; \s, \S: ANY ONE space/non-space character. Example of valid string are " a.1-2-3 ". Redirect from: Regex: /20[0-9][0-9]/.* Redirect to: Page: /blog/ I tested the above and all appears to be working as intended. However, I also understand that improper use of regex can cause problems for a site, so I’d really appreciate if you could please confirm if my regex is ok or if it should be edited in any way.

You could also use a regular expression that tests true for the cases that you don't want and then negate the result.

Cases you don't want:

  1. Starts with number or underscore: ^[0-9_]
  2. Consecutive underscores: __
  3. Something else than a letter, number or underscore: [^0-9a-z_]
  4. Ends with underscore: _$

Finally, join all of them with ORs |, giving:


And then negate the test:

!/^[0-9_]|_$|__|[^0-9a-z_]/gi.test('your string here')

See the tests here:

7.2. re — Regular expression operations — Python v3.1.5 , Regular expressions use the backslash character ('\') to indicate special forms or to As the target string is scanned, REs separated by '|' are tried from left to right. If the ASCII flag is used this becomes the equivalent of [^a-zA-Z0-9_] (but the� Check whether the regex is valid or not. We use cookies to ensure you have the best browsing experience on our website. Please read our cookie policy for more information about how we use cookies.

Can someone please explain this email validation rule into , OR maybe walk me through the code itself to explain what it's doing exactly? Not( OR(LEN(Additional_Email_Address__c )=0,(REGEX( a-zA-Z0-9 in the REGEX just means that any lower case alphabet character from "a� Regular Expression, or regex or regexp in short, is extremely and amazingly powerful in searching and manipulating text strings, particularly in processing text files. One line of regex can easily replace several dozen lines of programming codes.

10 Java Regular Expression Examples You Should Know, A-Za-z0-9]+ # follow by a dot "." and string in the bracket [ ], must contains one or more (+) )* # end of group #2, this group is optional (*) ( # start� Regular Expression Tester. This page uses the regular expression parser in your browser's implementation of JavaScript. This page should work with any browser that obeys the DOM, and Internet Explorer.* If you're looking for a .NET Regex tester, there's a crackerjack one here. Test String:

Url Validation Regex | Regular Expression - Taha Match or Validate phone number match whole word nginx test Match html tag Blocking site with unblocked games Find Substring within a string that begins and ends with paranthesis Empty String Checks the length of number and not starts with 0 Match dates (M/D/YY, M/D/YYY, MM/DD/YY, MM/DD/YYYY)