Regular Expressions 101

Explanation
Pro

Non-Capturing Group(?:<(\w+)[^>]*>(?:[\w+]+(?:(?!<).*?)<\/\1>?)[^\s\w]?|[^\s]+)

1st Alternative<(\w+)[^>]*>(?:[\w+]+(?:(?!<).*?)<\/\1>?)[^\s\w]?

<the literal < (60₁₀ / 74₈ / 3C₁₆)

Group 1(\w+)

\w+any word character, repeated at least once

Negated Character Class[^>]*

*matches any number of characters outside the set below:

>the literal > (62₁₀ / 76₈ / 3E₁₆)

Non-Capturing Group(?:[\w+]+(?:(?!<).*?)<\/\1>?)

Character Class[\w+]+

+matches at least one character from the set below:

\wany word character

+the literal + (43₁₀ / 53₈ / 2B₁₆)

Non-Capturing Group(?:(?!<).*?)

Negative Lookahead(?!<)

<the literal < (60₁₀ / 74₈ / 3C₁₆)

.*?any character (except for line terminators), repeated any number of times

<the literal < (60₁₀ / 74₈ / 3C₁₆)

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

\1the text saved by group 1

>?optionally matches the literal > (62₁₀ / 76₈ / 3E₁₆)

Negated Character Class[^\s\w]?

?matches at most one character outside the set below:

\sany whitespace character

\wany word character

2nd Alternative[^\s]+

Negated Character Class[^\s]+

+matches at least one character outside the set below:

\sany whitespace character

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

Detailed match information will be displayed here automatically.

25 tokens

(?:<(\w+)[^>]*>(?:[\w+]+(?:(?!<).*?)<\/\1>?)[^\s\w]?|[^\s]+)

New

A word, <a href='#' title=''>words within tags should remain intact</a>, so here's
<b>even more</b> <u>words</u> <span><b>ads</b>a</span>