You want to extract the fragment from a string that holds a URL. For example, you want to extract top from http://www.regexcookbook.com#top or from /index.html#top.

#(.+)

Extracting the fragment from a URL is trivial if you know that your subject text is a valid URL. The fragment is delimited from the part of the URL before it with a hash sign. The fragment is the only part of URLs in which hash signs are allowed, and the fragment is always the last part of the URL. Thus, we can easily extract the fragment by finding the first hash sign and grabbing everything until the end of the string. ‹#.+› does that nicely. Make sure to turn off free-spacing mode; otherwise, you need to escape the literal hash sign with a backslash.

This regular expression will find a match only for URLs that actually contain a fragment. The match consists of just the fragment, but includes the hash sign that delimits the fragment from the rest of the URL. The solution has an extra capturing group to retrieve just the fragment, without the delimiting #.

If you don’t already know that your subject text is a valid URL, you can use one of the regexes from Recipe 8.7. The first regex in that recipe captures the fragment, if one is present in the URL, into capturing group number 13.

Recipe 3.9 shows code to get the text matched by a particular part (capturing group) of a regex. Use this to get the fragment.

Techniques used in the regular expressions in this recipe are discussed in Chapter 2. Recipe 2.3 explains character classes. Recipe 2.5 explains anchors. Recipe 2.8 explains alternation. Recipe 2.9 explains grouping. Recipe 2.11 explains named capturing groups. Recipe 2.12 explains repetition. Recipe 2.18 explains how to add comments.