Regular Expressions 101

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/
^([^peE]*[eE][^peE]*){2,4}$
/
^ asserts position at start of the string
1st Capturing Group
([^peE]*[eE][^peE]*){2,4}
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
Match a single character not present in the list below
[^peE]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
peE
matches a single character in the list peE (case sensitive)
Match a single character present in the list below
[eE]
eE
matches a single character in the list eE (case sensitive)
Match a single character not present in the list below
[^peE]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
peE
matches a single character in the list peE (case sensitive)
$ asserts position at the end of the string, or before the line terminator right at the end of the string (if any)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String