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You wrote:

check if there is a digit or letter before the @@

I assume you mean the first @@.

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

Your regex has such an error that a dot matches any char. If you want to check for a literal dot, you must escape it with a backslash (compare with group 4 in my solution).

You wrote:

check if there is a digit or letter before the @@

I assume you mean a digit / letter before the first @@ and check for a dot after the second @@ (as in your example).

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

Your regex has such an error that a dot matches any char. If you want to check for a literal dot, you must escape it with a backslash (compare with group 4 in my solution).

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

Your regex has such an error that a dot matches any char. If you want to check for a literal dot, you must escape it with a backslash (compare with group 4 in my solution).

You wrote:

check if there is a digit or letter before the @@

I assume you mean the first @@.

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

Your regex has such an error that a dot matches any char. If you want to check for a literal dot, you must escape it with a backslash (compare with group 4 in my solution).

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

You can use the following regex:

[a-z0-9]+ # Chars before "@@", except the last (?: # Last char before "@@" (\d) # either a digit - group 1 | # or ([a-z]) # a letter - group 2 ) @@? # 1 or 2 "at" chars ([^@]+) # "Central" part - group 3 @@? # 1 or 2 "at" chars (?: # Check for a dot (\.) # Captured - group 4 | # or nothing captured ) [a-z0-9]+ # The last part # Flags: # i - case insensitive # x - ignore blanks and comments 

How it works:

• Group 1 or 2 captures the last char before the first @@ (either group 1 captures a digit or group 2 captures a letter).
• Group 3 catches the "central" part (THISSTRING, a sequence of chars other than @).
• Group 4 catches a dot, if any.

You can test it at https://regex101.com/r/ATjprp/1

Your regex has such an error that a dot matches any char. If you want to check for a literal dot, you must escape it with a backslash (compare with group 4 in my solution).