Regular Expressions 101

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"
^(?![^a-zA-Z0-9])(?:(?![.]{2,})[a-zA-Z0-9_\-.])+(?<![.])@(?:(?![^a-zA-Z0-9])(?:[a-zA-Z0-9_\-])+(?<![^a-zA-Z0-9])\.)+[a-zA-Z]{2,}$
"
gm
^ asserts position at start of a line
Negative Lookahead
(?![^a-zA-Z0-9])
Assert that the Regex below does not match
Match a single character not present in the list below
[^a-zA-Z0-9]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Non-capturing group
(?:(?![.]{2,})[a-zA-Z0-9_\-.])+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?![.]{2,})
Assert that the Regex below does not match
Match a single character present in the list below
[.]
{2,} matches the previous token between 2 and unlimited times, as many times as possible, giving back as needed (greedy)
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[a-zA-Z0-9_\-.]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Negative Lookbehind
(?<![.])
Assert that the Regex below does not match
Match a single character present in the list below
[.]
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
Non-capturing group
(?:(?![^a-zA-Z0-9])(?:[a-zA-Z0-9_\-])+(?<![^a-zA-Z0-9])\.)+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?![^a-zA-Z0-9])
Assert that the Regex below does not match
Match a single character not present in the list below
[^a-zA-Z0-9]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Non-capturing group
(?:[a-zA-Z0-9_\-])+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-zA-Z0-9_\-]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
Negative Lookbehind
(?<![^a-zA-Z0-9])
Assert that the Regex below does not match
Match a single character not present in the list below
[^a-zA-Z0-9]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[a-zA-Z]
{2,} matches the previous token between 2 and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.

Regular Expression
No Match

r"
"
gm

Test String