Regular Expressions 101

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/
^([a-z0-9\._]+)@([a-z]+)\.([a-z]{2,4})
/
gm
^ asserts position at start of a line
1st Capturing Group
([a-z0-9\._]+)
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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
2nd Capturing Group
([a-z]+)
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)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
3rd Capturing Group
([a-z]{2,4})
Match a single character present in the list below
[a-z]
{2,4} matches the previous token between 2 and 4 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)
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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String