visual basic 6.0 barcode generator 6: Crawling the Web with Java in Java

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6: Crawling the Web with Java
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You can obtain a string containing a matching sequence by calling group( ). The form used by Search Crawler is shown here: String group(int which) Here, which specifies the sequence (group of characters), with the first group being 1. The matching string is returned.
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The syntax and rules that define a regular expression are similar to those used by Perl 5. Although no single rule is complicated, there are a large number of them, and a complete discussion is beyond the scope of this book. However, a few of the more commonly used constructs are described here. In general, a regular expression is comprised of normal characters, character classes (sets of characters), wildcard characters, and quantifiers. A normal character is matched as is. Thus, if a pattern consists of "xy", the only input sequence that will match it is "xy". Characters such as newlines and tabs are specified using the standard escape sequences, which begin with a backslash (\). For example, a newline is specified by \n. In the language of regular expressions, a normal character is also called a literal. A character class is a set of characters. A character class is specified by putting the characters in the class between brackets. For example, the class [wxyz] matches w, x, y, or z. To specify an inverted set, precede the characters with a circumflex (^). For example, [^wxyz] matches any character except w, x, y, or z. You can specify a range of characters using a hyphen. For example, to specify a character class that will match the digits 1 through 9, use [1 9]. The wildcard character is the dot (.), and it matches any character. Thus, a pattern that consists of "." will match these (and other) input sequences: "A", "a", "x", and so on. A quantifier determines how many times an expression is matched. The quantifiers are shown here:
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+ * Match one or more. Match zero or more. Match zero or one.
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For example, the pattern "x+" will match "x", "xx", and "xxx", among others.
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A Close Look at retrieveLinks( )
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The retrieveLinks( ) method uses the regular expression API to obtain the links from a page. It begins with these lines of code:
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// Compile link matching pattern. Pattern p = Pattern.compile("<a\\s+href\\s*=\\s*\" (.* )[\"|>]", Pattern.CASE_INSENSITIVE); Matcher m = p.matcher(pageContents);
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The regular expression used to obtain links can be broken down as a series of steps, as shown in the following table: Character Sequence
<a \\s+ href \\s* = \\s* \" (.* ) [\ |>]
Explanation
Look for the characters "<a". Look for one or more space characters. Look for the characters href . Look for zero or more space characters. Look for the character "=". Look for zero or more space characters. Look for zero or one quote character. Look for zero or more of any character until the next part of the pattern is matched, and place the results in a group. Look for quote character or greater than (">") character.
Notice that Pattern.CASE_INSENSITIVE is passed to the pattern compiler. As mentioned, this indicates that the pattern should ignore case when searching for matches. Next, a list to hold the links is created, and the search for the links begins, as shown here:
// Create list of link matches. ArrayList linkList = new ArrayList(); while (m.find()) { String link = m.group(1).trim();
Each link is found by cycling through m with a while loop. The find( ) method of Matcher returns true until no more matches are found. Each match (link) found is retrieved by calling the group( ) method defined by Matcher. Notice that group( ) takes 1 as an argument. This specifies that the first group from the matching sequences be returned. Notice also that trim( ) is called on the return value from the group( ) method. This removes any unnecessary leading or trailing space from the value. Many of the links found in Web pages are not suited for crawling. The following code filters out several links that the Search Crawler is uninterested in:
// Skip empty links. if (link.length() < 1) { continue; } // Skip links that are just page anchors. if (link.charAt(0) == '#') { continue; } // Skip mailto links.
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