I want to understand regular expressions that check HTML start and end tags.

Hello, I think you should copy the regular expression on this site and read the analysis results

https://regexr.com/

If you can read English, you will understand it as it is.

After a match to <, the first open bracket allows spaces, the second open bracket allows spaces after matching to recognize both closed open tags.The tag name is followed by the tag name, and the third open bracket is followed by a blank or non-> character.After the fourth open bracket, close > while making sure that it does not precede a new line <

I think it's like that.

First of all, the regular expression you wrote seems to be embedded in the Java string literal.You already understand that the string literal "\\" in Java represents one character of \, but \ is also meaningful in the Markdown notation used here in the stack overflow, so it appears as \s or mixed with \s.

In the following description,

• Make sure to add " on both sides to indicate that it is a notation in the string literal
  (Fortunately, " does not appear in this regular expression, so I think it will be difficult to get confused.)
• The characters themselves are not in code style or bold
• Metacharacters (symbols that represent non-characters themselves) are bold in their code style

If you reshape the regular expression in your question with the above rules, it looks like this.

"((\\s*(?:\\s*)?[tag name](?:\\s*|\\s+[^+))(?:>|(?=)|$(?!\\n))"

(There will be gaps in strange places, but there will be no intentional spaces.)

It would be nice if the browser you are using showed it in an easy-to-read way, but there are only three types of characters that represent </> characters themselves

All other characters are metacharacters, meaning something.All meta-characters that can be used in Java's regular expressions are listed in Java's official document.(However, it's not your fault to think "I don't know!" because you can't say it's easy to understand anywhere.Please read it in conjunction with other explanatory articles.Also, there may be some changes depending on the Java version.)

Pattern (Java platform SE8)

Among them, we will list the regular expressions in your question by applying them to the previous rule and shaping them.

"XX, forward-referencing regular expression group

"\\s" blank characters:[\t\n\x0B\f\r]

"X*"X, 0 or more

"(?:X)"X, regular expression group without forward reference

"X?"X, 1 or 0 times

"X|YX or Y

"X+X, at least once

"[^abc]""" Non-a, b, c characters (denial)

"(?=X)"X, affirmative first reading of zero width

"$" end of line

"(?!X)"X, negative read to zero width

"\\n" Newline Characters ("\u000A")

• Many commentary articles use the word "capture" for "forward reference."
• As for "zero width" and "forward reading," it means that it checks if it matches, but the check position cannot be advanced."If you search for ""positive regular expressions"" or something like that, you will find explanatory articles that are easy to read."

As for ~ to に in the postscript, if you modify (?:) by dividing it into parentheses and ?:, you will not be able to mention it individually.Please read this answer (or link) and let us know if you have any questions.