juneau-core/juneau-marshall/src/main/java/org/apache/juneau/svl/vars/PatternExtractVar.java

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package org.apache.juneau.svl.vars;

import static org.apache.juneau.internal.ThrowableUtils.*;

import java.util.regex.*;

import org.apache.juneau.svl.*;

/**
 * A transformational variable that returns matched regex groups by given index.
 *
 * <p>
 * The format for this var:
 * <ul>
 * 	<li><js>"$PE{arg,pattern,groupIndex}"</js>
 * </ul>
 *
 * <p>
 * The pattern can be any string optionally containing <js>'*'</js> or <js>'?'</js> representing any or one character
 * respectively.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bcode w800'>
 * 	<jc>// Create a variable resolver that resolves system properties and $SW vars.</jc>
 * 	VarResolver r = VarResolver.<jsm>create</jsm>().vars(PatternExtractVar.<jk>class</jk>, SystemPropertiesVar.<jk>class</jk>).build();
 *
 * 	<jc>// Use it!</jc>
 * 	System.<jsf>out</jsf>.println(r.resolve(<js>"Update = $PE{$P{java.version},_([0-9]+),1}!"</js>));
 * </p>
 *
 * <p>
 * Since this is a {@link MultipartVar}, any variables contained in the result will be recursively resolved.
 * <br>Likewise, if the arguments contain any variables, those will be resolved before they are passed to this var.
 *
 * <ul class='seealso'>
 * 	<li class='link'>{@doc juneau-svl.SvlVariables}
 * </ul>
 */
public class PatternExtractVar extends MultipartVar {

	/** The name of this variable. */
	public static final String NAME = "PE";

	/**
	 * Constructor.
	 */
	public PatternExtractVar() {
		super(NAME);
	}

	@Override /* MultipartVar */
	public String resolve(VarResolverSession session, String[] args) {
		if (args.length < 3)
			illegalArg("Invalid number of arguments passed to $PE var.  Must have 3 arguments.");

		String stringArg = args[0];
		String pattern = args[1];
		String result = "";
		int groupId = Integer.parseInt(args[2]);

		Pattern p = Pattern.compile(pattern.replace("*", ".*").replace("?", "."));
		Matcher m = p.matcher(stringArg);

		if (m.find() && groupId <= m.groupCount() && groupId >= 0) {
			result = m.group(groupId);
		}

		return result;
	}
}