Regular Expressions 101

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/
(?:^| ).{6}[^ \n]*(?= )
/
gm
Non-capturing group
(?:^| )
1st Alternative
^
^ asserts position at start of a line
2nd Alternative
matches the character with index 3210 (2016 or 408) literally (case sensitive)
.
matches any character (except for line terminators)
{6} matches the previous token exactly 6 times
Match a single character not present in the list below
[^ \n]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
matches the character with index 3210 (2016 or 408) literally (case sensitive)
\n matches a line-feed (newline) character (ASCII 10)
Positive Lookahead
(?= )
Assert that the Regex below matches
matches the character with index 3210 (2016 or 408) literally (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.

Regular Expression
No Match

/
/
gm

Test String