RegexBuddy explanation.

^[A-Za-z0-9-_.+%]+@[A-Za-z0-9-.]+\.[A-Za-z]{2,4}$

Options: case insensitive; ^ and $ match at line breaks

Assert position at the beginning of a line (at beginning of the string or after a line break character) «^»
Match a single character present in the list below «[A-Za-z0-9-_.+%]+»
   Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
   A character in the range between “A” and “Z” «A-Z»
   A character in the range between “a” and “z” «a-z»
   A character in the range between “0” and “9” «0-9»
   One of the characters “-_.+%” «-_.+%»
Match the character “@” literally «@»
Match a single character present in the list below «[A-Za-z0-9-.]+»
   Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
   A character in the range between “A” and “Z” «A-Z»
   A character in the range between “a” and “z” «a-z»
   A character in the range between “0” and “9” «0-9»
   One of the characters “-.” «-.»
Match the character “.” literally «\.»
Match a single character present in the list below «[A-Za-z]{2,4}»
   Between 2 and 4 times, as many times as possible, giving back as needed (greedy) «{2,4}»
   A character in the range between “A” and “Z” «A-Z»
   A character in the range between “a” and “z” «a-z»
Assert position at the end of a line (at the end of the string or before a line break character) «$»


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