Delphi XE2正则表达式：量化器在正向后方内部不起作用？

Question

I have a complete HTML document string from a web page containing this BASE tag:

<BASE href="http://whatreallyhappened.com/">

In Delphi XE2, I use this regular expression with the whole HTML document as subject to get the URL from the BASE tag between the double quotes:

BaseURL := TRegEx.Match(HTMLDocStr, '(?<=<base(\s)href=").*(?=")', [roIgnoreCase]).Value;

This works, but only if there is only ONE space character in the subject between BASE and href .

I tried to add a quantifier to the space part in the regex (\\s) , but it did not work.

So how can I make this regex match the URL even if there are several spaces between BASE and href ?

Answer 1

You're making this far too complicated by using lookaround. If you want to extract only part of the regex match, simply add a capturing group. Then you can use the text matched by the capturing group instead of the overall match. In most cases you'll also get much better performance this way.

To find the base tag in a file and extract its URL you can use the regex <base[^>]+href=["']([^"']*)["'] . Call TRegex.Match() to get a TMatch . This has a Groups property that you can use to retrieve group 1 if a match was found.

Answer 2

With lookaround

You can use different ways to try using quantifiers like these:

(?<=<BASE)\s+href=".*(?=")
(?<=<BASE)\s{0,30}href=".*(?=")

Working demo

Without lookaround

By the way, if you want just to get the content within href there is no need of lookaround you just can use:

<BASE\s+href="(.*?)"

Working demo

EDIT: after reading your comments I figured out a workaround (ugly but could work). You can try using something like this:

((?<=<BASE\shref=")|(?<=<BASE\s\shref=")|(?<=<BASE\s\s\shref=")).*(?=")
          ^---notice \s        ^---notice \s\s       ^---notice \s\s\s

I know that this is horrible, but if none of above work you can try with that.