Email Regular Expression Explanation - regex

Question

I'm trying to understand a regular expression which is currently being used to validate the input of an email address on a website. The value of this email address is used to populate a target system; validation of which can be expressed in plain English.
I would like to be able to highlight, with the use of examples, where the website validated email address imposes validation rules that are not required in the target system. To this end, I have obtained the regular expression from the developer, and am requiring some assistance in translating it to allow it to be understood in plain English:
^[_A-Za-z0-9_%+-]+(\\.[_A-Za-z0-9_%+-]+)*#[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,4})$
So far, I have gained some understanding from a previous post.
... which would seem to confirm the following:
^ = The matched string must begin here, and only begin here
[ ] = match any character inside the brackets, but only match one.
I'm not sure of the relevance of "only match one". Can anyone advise?
\+ = match previous expression at least once, unlimited number of times.
Presumably this means the previous expression refers to the characters contained within the preceding square brackets and it can be repeated unlimited times?
() = make everything inside the parentheses a group (and make them referencable).
I'm not sure what this might mean.
\\. = match a literal full stop (.)
Then we have a repeat of the square bracket content, though I'm unsure what the relevance is here since the initial square brackets character class can be repeated unlimited times?
# = match a literal # symbol
The final parenthesis seems to match the top level domain which must be at least 2 characters but no more than 4 characters.
I think my main issue is in understanding the relevance of the round brackets as I can't understand what they add beyond what the square brackets add.
Any help would be much appreciated.

Answer 1

^[_A-Za-z0-9_%+-]+(\\.[_A-Za-z0-9_%+-]+)*#[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,4})$
Assert position at the beginning of the string «^»
Match a single character present in the list below «[_A-Za-z0-9_%+-]+»
Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
The character “_” «_»
A character in the range between “A” and “Z” «A-Z»
A character in the range between “a” and “z” «a-z»
A character in the range between “0” and “9” «0-9»
One of the characters “_%” «_%»
The character “+” «+»
The character “-” «-»
Match the regular expression below and capture its match into backreference number 1 «(\\.[_A-Za-z0-9_%+-]+)*»
Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
Note: You repeated the capturing group itself. The group will capture only the last iteration. Put a capturing group around the repeated group to capture all iterations. «*»
Match the character “\” literally «\\»
Match any single character that is not a line break character «.»
Match a single character present in the list below «[_A-Za-z0-9_%+-]+»
Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
The character “_” «_»
A character in the range between “A” and “Z” «A-Z»
A character in the range between “a” and “z” «a-z»
A character in the range between “0” and “9” «0-9»
One of the characters “_%” «_%»
The character “+” «+»
The character “-” «-»
Match the character “#” literally «#»
Match a single character present in the list below «[A-Za-z0-9]+»
Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
A character in the range between “A” and “Z” «A-Z»
A character in the range between “a” and “z” «a-z»
A character in the range between “0” and “9” «0-9»
Match the regular expression below and capture its match into backreference number 2 «(\\.[A-Za-z0-9]+)*»
Between zero and unlimited times, as many times as possible, giving back as needed (greedy) «*»
Note: You repeated the capturing group itself. The group will capture only the last iteration. Put a capturing group around the repeated group to capture all iterations. «*»
Match the character “\” literally «\\»
Match any single character that is not a line break character «.»
Match a single character present in the list below «[A-Za-z0-9]+»
Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+»
A character in the range between “A” and “Z” «A-Z»
A character in the range between “a” and “z” «a-z»
A character in the range between “0” and “9” «0-9»
Match the regular expression below and capture its match into backreference number 3 «(\\.[A-Za-z]{2,4})»
Match the character “\” literally «\\»
Match any single character that is not a line break character «.»
Match a single character present in the list below «[A-Za-z]{2,4}»
Between 2 and 4 times, as many times as possible, giving back as needed (greedy) «{2,4}»
A character in the range between “A” and “Z” «A-Z»
A character in the range between “a” and “z” «a-z»
Assert position at the end of the string (or before the line break at the end of the string, if any) «$»