Regular Expressions 101

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/
^(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9]).{8,}$
/
gm
^ asserts position at start of a line
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)
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
(?=.*[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)
.
matches any character (except for line terminators)
{8,} matches the previous token between 8 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