需要正则表达式建议

Question

<img[^>]+src\\s*=\\s*['\"]([^'\"]+)['\"][^>]*>

I know this regex expression is used to retrieve the value of src. Can anyone teach me how i should interpret this expression? stucked at it.

Answer 1

Explaining:

• <img matches exactly the string "<img"
• [^>]+ matches multiple times of everything but > , so the tag will not be closed
• src matches exactly the string "src"
• \\\\s* matches any number of whitespace characters
• = matches exactly the string "="
• \\\\s* matches any number of whitespace characters
• ['\\"] matches the two quotes. The double quote is escaped, because otherwise it will terminate the string of the regex
• ([^'\\"]+) mathches multiple times everything but quotes. The contents are wrapped in brackets, so that they are declared as group and can be retrieved later
• ['\\"] matches the two quotes. The double quote is escaped, because otherwise it will terminate the string of the regex
• [^>]* matches the remaining non ">" characters
• > matches exactly the string ">" , the closing bracket of the tag.

I would not agree this expression is a crap, just a bit complex.

EDIT Here you go some examplary code:

String str = "<img[^>]+src\\s*=\\s*['\"]([^'\"]+)['\"][^>]*>";
String text = "<img alt=\"booo\" src=\"image.jpg\"/>";
Pattern pattern = Pattern.compile (str);
Matcher matcher = pattern.matcher (text);

if (matcher.matches ())
{
      int n = matcher.groupCount ();
      for (int i = 0; i <= n; ++i)
          System.out.println (matcher.group (i));
}

The output is:

<img alt="booo" src="image.jpg"/>
image.jpg

So matcher.group(1) returns what you want. experiment a bit with this code.

Answer 2

Hi check one of the tutorials available on the net - eg http://www.vogella.com/articles/JavaRegularExpressions/article.html . Section 3.1 and 3.2 common matching symbols explains briefly each symbol and what it replaces as well as metacharacters. Break what you have here into smaller chunks to understand it easier. For example you have \\s in two places it is a metacharacter for a whitespace character. Backslash is an escape character in Java thus you have \\s instead of \\s. After each of them you have a . Section 3.3 explains the quantifiers - this particular one means it occurs 0 or more times. Thus the \\s means "search for a whitespace character that occurs 0 or more times". You do the same with other chunks.

Hope it helps.