Regular Expressions 101

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/
^(0|[1-9]+[0-9]*) (@[^@]+@ |)([A-Za-z0-9_]+)( [^\n\r]*|)$
/
gm
^ asserts position at start of a line
1st Capturing Group
(0|[1-9]+[0-9]*)
1st Alternative
0
0 matches the character 0 with index 4810 (3016 or 608) literally (case sensitive)
2nd Alternative
[1-9]+[0-9]*
Match a single character present in the list below
[1-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
1-9 matches a single character in the range between 1 (index 49) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
2nd Capturing Group
(@[^@]+@ |)
1st Alternative
@[^@]+@
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
Match a single character not present in the list below
[^@]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
@ matches the character @ with index 6410 (4016 or 1008) literally (case sensitive)
@
matches the characters @ literally (case sensitive)
2nd Alternative null, matches any position
3rd Capturing Group
([A-Za-z0-9_]+)
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 65) and Z (index 90) (case sensitive)
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 9510 (5F16 or 1378) literally (case sensitive)
4th Capturing Group
( [^\n\r]*|)
1st Alternative
[^\n\r]*
matches the character with index 3210 (2016 or 408) literally (case sensitive)
Match a single character not present in the list below
[^\n\r]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\n matches a line-feed (newline) character (ASCII 10)
\r matches a carriage return (ASCII 13)
2nd Alternative null, matches any position
$ 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.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String