Internationalized domain names email validation regex

A very specific and interesting topic!

Internationalized Domain Names (IDNs) allow domain names to contain non-ASCII characters, such as accented letters, non-Latin scripts, and non-English characters. To validate an email address that may contain IDNs, you'll need a regular expression that can match a wide range of characters.

Here's a regex pattern that should help you validate an email address with IDNs:

^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\.?([a-zA-Z]{2,}|xn--[a-zA-Z0-9]{1,59})$

Let's break down this regex pattern:

Local part (^[a-zA-Z0-9.!#$%&'*+/=?^_{|}~-]+`):

• ^ matches the start of the string
• [a-zA-Z0-9.!#$%&'*+/=?^_{|}~-]+` matches one or more of the following characters:
  • Letters (both ASCII and non-ASCII, thanks to the Unicode property \p{L})
  • Digits
  • Special characters (.,!, #, $, %, &, *, +, /, =,?, ^, _, `, {, |, ~)
• + quantifier matches one or more of the preceding pattern

Domain (@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\.?):

• @ matches the @ symbol
• [a-zA-Z0-9] matches a single letter or digit (the first character of the domain)
• (?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])? matches an optional sequence of:
  • 0 to 61 occurrences of letters, digits, or hyphens
  • followed by a letter or digit
• (?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)* matches an optional sequence of:
  • a dot (\.)
  • followed by a domain (matching the same pattern as above)
  • repeated 0 or more times
• \.? matches an optional dot (\.)

Top-level domain (([a-zA-Z]{2,}|xn--[a-zA-Z0-9]{1,59})$):

• ([a-zA-Z]{2,}|xn--[a-zA-Z0-9]{1,59}) matches the top-level domain (TLD)
  • ([a-zA-Z]{2,}) matches 2 or more ASCII letters (e.g., "com", "org")
  • | is an OR operator
  • xn-- is a prefix for IDN TLDs (e.g., "xn--wgbh1c3ax7g")
  • [a-zA-Z0-9]{1,59} matches 1 to 59 occurrences of letters, digits, or hyphens
• $ matches the end of the string

This regex pattern should match most valid email addresses, including those with IDNs. However, keep in mind that email address validation is a complex topic, and this pattern may not cover all possible valid email addresses.

Remember to adjust the regex pattern according to your specific requirements and the programming language you're using.