^start of line
Positive Lookahead(?=.* (?!.* ))? ?repeats the group contents below at most once:
.*any character (except for line terminators), repeated any number of times
the literal (3210 / 408 / 2016)
Negative Lookahead(?!.* ) .*any character (except for line terminators), repeated any number of times
the literal (3210 / 408 / 2016)
athe literal a (9710 / 1418 / 6116) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
bthe literal b (9810 / 1428 / 6216) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
cthe literal c (9910 / 1438 / 6316) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
dthe literal d (10010 / 1448 / 6416) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
ethe literal e (10110 / 1458 / 6516) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
fthe literal f (10210 / 1468 / 6616) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
gthe literal g (10310 / 1478 / 6716) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
hthe literal h (10410 / 1508 / 6816) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
ithe literal i (10510 / 1518 / 6916) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
jthe literal j (10610 / 1528 / 6A16) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
kthe literal k (10710 / 1538 / 6B16) (case sensitive)
?optionally matches the literal (3210 / 408 / 2016)
$end of line
g modifier: global. Finds all matches instead of stopping after the first
m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string