Regular Expressions 101

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/
(?=[A-Z][A-Za-z])(?i)(?!.*[aeiouy]{3}|.*[^aeiouy]{3})[a-z]+
/
gm
Positive Lookahead
(?=[A-Z][A-Za-z])
Assert that the Regex below matches
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)
Match a single character present in the list below
[A-Za-z]
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)
(?i)
match the remainder of the pattern with the following effective flags: gmi
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Negative Lookahead
(?!.*[aeiouy]{3}|.*[^aeiouy]{3})
Assert that the Regex below does not match
1st Alternative
.*[aeiouy]{3}
.
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
[aeiouy]
{3} matches the previous token exactly 3 times
aeiouy
matches a single character in the list aeiouy (case insensitive)
2nd Alternative
.*[^aeiouy]{3}
.
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 not present in the list below
[^aeiouy]
{3} matches the previous token exactly 3 times
aeiouy
matches a single character in the list aeiouy (case insensitive)
Match a single character present in the list below
[a-z]
+ 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 97) and z (index 122) (case insensitive)
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