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I have a Regex to allow alphanumeric, underscore and dots but not consecutive dots:
^(?!.*?[.]{2})[a-zA-Z0-9_.]+$
I also need to now allow dots in the first and last character of the string.
How can I do this?
829
(dot) metacharacter, and can match any single character (letter, digit, whitespace, everything). You may notice that this actually overrides the matching of the period character, so in order to specifically match a period, you need to escape the dot by using a slash \.
The backslash in combination with a literal character can create a regex token with a special meaning. E.g. \d is a shorthand that matches a single digit from 0 to 9. Escaping a single metacharacter with a backslash works in all regular expression flavors.
Special characters such as the dot character often need to be escaped in a regex pattern if you want to match them. For example, to match the actual dot '.
You can use it like this with additional lookaheads:
^(?!\.)(?!.*\.$)(?!.*?\.\.)[a-zA-Z0-9_.]+$
(?!\.) - don't allow . at start(?!.*?\.\.) - don't allow 2 consecutive dots (?!.*\.$) - don't allow . at end
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Re-write the regex as
^[a-zA-Z0-9_]+(?:\.[a-zA-Z0-9_]+)*$
or (in case your regex flavor is ECMAScript compliant where \w = [a-zA-Z0-9_]):
^\w+(?:\.\w+)*$
See the regex demo
Details:
^ - start of string[a-zA-Z0-9_]+ - 1 or more word chars (?:\.[a-zA-Z0-9_]+)* - zero or more sequences of:
\. - a dot[a-zA-Z0-9_]+ - 1 or more word chars$ - end of string
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