How can regular expressions be used to prevent certain special characters from being used at the beginning or end of an email username in an email address?

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To prevent certain special characters from being used at the beginning or end of an email username in an email address, the following regular expression pattern can be used:

^(?![_\-.])\w+(?<![_\-.])@[a-zA-Z0-9]+\.[a-zA-Z]{2,3}$

Explanation of the pattern:

• ^ - start of the string
• (?![_\-.]) - negative lookahead assertion to prevent the use of underscore _, hyphen -, and dot . at the beginning of the username
• \w+ - one or more word characters (letters, digits, and underscore) for the username
• (?<![_\-.]) - negative lookbehind assertion to prevent the use of underscore, hyphen, and dot at the end of the username
• @ - the at symbol, which separates the username and domain name parts of the email address
• [a-zA-Z0-9]+\.[a-zA-Z]{2,3} - the domain name consisting of one or more letters and digits followed by a dot and a two- or three-letter top-level domain (TLD)
• $ - end of the string

This regular expression pattern ensures that the email username does not start or end with underscore, hyphen, or dot symbols.