Regular Expressions 101

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"
^(?=.*[~!@#$%^&*()_+`\-=\[\]\{\};':\",./<>?])(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z])\S{6,}$
"
gm
^ asserts position at start of a line
Positive Lookahead
(?=.*[~!@#$%^&*()_+`\-=\[\]\{\};':\",./<>?])
Assert that the Regex below matches
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[~!@#$%^&*()_+`\-=\[\]\{\};':\",./<>?]
~!@#$%^&*()_+`
matches a single character in the list ~!@#$%^&*()_+` (case sensitive)
\- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
= matches the character = with index 6110 (3D16 or 758) literally (case sensitive)
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
\] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
\{ matches the character { with index 12310 (7B16 or 1738) literally (case sensitive)
\} matches the character } with index 12510 (7D16 or 1758) literally (case sensitive)
;':
matches a single character in the list ;': (case sensitive)
\" matches the character " with index 3410 (2216 or 428) literally (case sensitive)
,./<>?
matches a single character in the list ,./<>? (case sensitive)
Positive Lookahead
(?=.*[0-9])
Assert that the Regex below matches
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[0-9]
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Positive Lookahead
(?=.*[a-z])
Assert that the Regex below matches
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Positive Lookahead
(?=.*[A-Z])
Assert that the Regex below matches
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\S
matches any non-whitespace character (equivalent to [^\r\n\t\f\v ])
{6,} matches the previous token between 6 and unlimited times, as many times as possible, giving back as needed (greedy)
$ 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.

Regular Expression
No Match

"
"
gm

Test String