Regular Expressions find and replace

Question

Hi ,
I have a question on using Regular Expressions in Java(java.util.regex).
Problem Description:
I have a string (say for example strHTML) which contains the whole HTML code of a webpage. I want to be able to search for all the image source tags and check whether they are absolute urls to the image source(for eg. <img src="www.google.com/images/logo.gif" >) or relative(for eg. <img src="../images/logo.gif" >).
If they are realtive urls to the image path, then I wish to replace them with their absolute urls throughout the webpage(in this case inside string strHTML).
I have to do it inside a servlet and hence have to use java.
I tried . This is the code. It doesn't match and replace and goes inside an infinite loop i.e probably the pattern matches everything.
//Change all images to actual http addresses FOR example change src="../images/logo.gif" to src="http://www.google.com/../images/logo.gif"
          String ddurl="http://www.google.com/";
String strHTML=" < img src=\"../images/logo.gif\" alt=\"Google logo\">";
Pattern p = Pattern.compile ("(?i)src[\\s]*=[\\s]*[\"\']([./]*.*)[\"\']");
Matcher m = p.matcher (strHTML);
while(m.find())
m.replaceAll(ddurl+m.group(1));
what is wrong in this?
Thanks,
Rajiv

Answer 1

Right, here's the full monte (whatever that means):import java.util.regex.*;
public class Test1
  public static void main(String[] args)
    String domain = "http://www.google.com/";
    String strHTML =
      " < img src=\"images/logo.gif\" alt=\"Google logo\">\n" +
      " <img alt=\"Google logo\" src=images/logo.gif >\n" +
      " <IMG SRC=\"/images/logo.gif\" alt=\"Google logo\">\n" +
      " <img alt=\"Google logo\" src=../images/logo.gif>\n" +
      " <img src=http://www.yahoo.com/images/logo.gif alt=\"Yahoo logo\">";
    String regex =
      "(<\\s*img.+?src\\s*=\\s*)   # Capture preliminaries in $1.  \n" +
      "(?:                         # First look for URL in quotes. \n" +
      "   ([\"\'])                 #   Capture open quote in $2.   \n" +
      "   (?!http:)                #   If it isn't absolute...     \n" +
      "   /?(.+?)                  #    ...capture URL in $3       \n" +
      "   \\2                      #   Match the closing quote     \n" +
      " |                          # Look for non-quoted URL.      \n" +
      "   (?!http:)                #   If it isn't absolute...     \n" +
      "   /?([^\\s>]+)             #    ...capture URL in $4       \n" +
    Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE | Pattern.COMMENTS);
    Matcher m = p.matcher(strHTML);
    StringBuffer sbuf = new StringBuffer();
    while (m.find())
      String relURL = m.group(3) != null ? m.group(3) : m.group(4);
      m.appendReplacement(sbuf, "$1\"" + domain + relURL + "\"");
    m.appendTail(sbuf);
    System.out.println(sbuf.toString());
}First off, observe that I'm using free-spacing (or "COMMENTS") mode to make the regex easier to read--all the whitespace and comments will be ignored by the Pattern compiler. I also used the CASE_INSENSITIVE flag instead of an embedded (?i), just to remove some clutter. By the way, your second (?i) was redundant; the first one would remain in effect until "turned off" with a (?-i). Another way to localize a flag's effect by using it within a non-capturing group, e.g., (?i:img).
As jaylogan said, the best way to filter out absolute URL's is by using a negative lookahead, and that's what I've done here. The problem of optional quotes I addressed by trying to match first with quotes, then without. The all-in-one approach might work with URL's, since they can't (AFAIK) contain whitespace anyway, but the alternation method can be used to match any attribute/value pair. It's also, I feel, easier to understand and maintain. Unfortunately, it also means that you can't use replaceAll(), since you have to determine which alternative matched before doing the replacement, but the long version is still pretty simple (especially when you can just copy it from the javadoc for the appendReplacement() method, as I did).