Regular Expressions 101

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/
(^[a-zA-Z0-9_.]+[@]{1}[a-z0-9]+[\.][a-z]+$)
/
mg
1st Capturing Group
(^[a-zA-Z0-9_.]+[@]{1}[a-z0-9]+[\.][a-z]+$)
^ asserts position at start of a line
Match a single character present in the list below
[a-zA-Z0-9_.]
+ matches the previous token between one 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)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
_.
matches a single character in the list _. (case sensitive)
Match a single character present in the list below
[@]
{1} matches the previous token exactly one time (meaningless quantifier)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one 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)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[\.]
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Match a single character present in the list below
[a-z]
+ matches the previous token between one 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)
$ asserts position at the end of a line
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
mg

Test String