Please explain this regex

I have regex whi开发者_StackOverflow社区ch reads:

@"<img\s*[^>]*>(?:\s*?</img>)?

Can someone please explain this part: (?:\s*?)?

What is that?

match but don't capture any number of whitespace followed by a close image tag, zero or one times:

(?: = match but don't capture

\s*? = any number of whitespace (not greedy)

</img> = close image tag

)? = zero or one times

:)

(?:\s*?) selects any whitespace, if it exists, after the image tag. The ?: at the beginning tells the regex engine to not capture that group (meaning it won't be returned in the matches array)

non-capturing group of any number of whitespace characters, followed by a closing img tag

The entire expression will capture any <img> tags that have corresponding </img> tags (but it won't capture the close tags). It doesn't capture the close tags because the (?:) syntax means "match but don't capture".

Some restrictions that are part of this regex:

1. The \s* in the opening tag is redundant because [^>]* will capture this too
2. Only whitespace is allowed between the opening and closing tags

Some examples:

• <img> will not match
• <img></img> will match, but only capture <img>
• <img attr="123"></img> will match, but only capture <img attr="123">
• <imgabc></img> will not match
• <img> </img> will match, but only capture <img>
• <img>ab</img> will not match

I highly recommend the Regular Expression Designer available for free at www.radsoftware.com.au for testing regexs