This regular expression has the following features that make it a convenient tool for working with text data.
(?=\S) Positive lookahead, trims leading whitespace in text block.
(?<text> ... ) Group for text block, which can be comment, section, entry, or undefined string.
(?<comment> ... ) Group for comment. Includes # or ;, then spaces (if any), then comment value.
(?<open>[#;]+) Group for comment opening characters (# or ;).
(?:[^\S\r\n]*) Non-capturing group for spaces, excluding newlines.
(?<value>.+) Group for value following comment opening characters.
(?<section> ... ) Group for section. Includes opening bracket [, then spaces (if any), then section value, then closing bracket ].
(?<open>[) Group for opening bracket [.
(?:\s*) Non-capturing group for spaces after the opening bracket [.
(?<value>[^]]*\S+) Group for the section value, excluding the closing bracket ] and capturing the last non-space character.
(?:[^\S\r\n]*) Non-capturing group for spaces after the section value before the closing bracket ].
(?<close>]) Group for the closing bracket ].
(?<entry> ... ) Group for an entry (parameter and its value). Includes the key, the separator (: or =), and the value.
(?<key>[^=\r\n[]]*\S) Group for the entry key, excluding the =, [, ] and newline characters, and capturing the last non-space character.
(?:[^\S\r\n]*) Non-capturing group for spaces after the key before the separator (: or =).
(?<delimiter>: =)
(?<value>[^#;\r\n]*) Group for the entry value, excluding #, ; and newline.
(?:[^\S\r\n]*) Non-capturing group for whitespace after the entry value.
(?<undefined>.+) Group for an undefined string that does not match any other rules.
(?<=\S) Positive lookahead, trims whitespace at the end of a text block.
`(?<linebreaker>\r\n|\n) Group for newline characters.
(?<whitespace>[^\S\r\n]+) Group for whitespace characters, excluding newline characters.