What causes "regex must not be empty" in code4craft/webmagic?

Supply a non-blank regex string to the RegexSelector constructor

code4craft/webmagic · error · IllegalArgumentException

regex must not be empty

Error message

regex must not be empty

What it means

RegexSelector validates its pattern in compileRegex and throws IllegalArgumentException when the regex string is null, empty, or blank. A regex selector without a pattern cannot match anything, so construction is refused immediately.

Solutions

Supply a non-blank regex string to the RegexSelector constructor
Validate the pattern with StringUtils.isNotBlank(regex) before constructing the selector
Fix the config/source that yields the empty value

Example fix

// before
Selectable sel = new RegexSelector("");
// after
Selectable sel = new RegexSelector("<title>(.*?)</title>");

Defensive patterns

Strategy: validation

Validate before calling

if (regexStr == null || regexStr.trim().isEmpty()) { throw new IllegalArgumentException("regex required"); }

Try / catch

try { sel = new RegexSelector(cfgRegex); } catch (IllegalArgumentException e) { log.error("blank/invalid regex config", e); sel = defaultSelector; }

Prevention

Validate regex config values at startup with StringUtils.isNotBlank
Fail fast on empty settings files
Provide sane default patterns

When it happens

Trigger: new RegexSelector("") or new RegexSelector(null); passing an empty regexStr read from blank configuration; RegexSelector created with a blank string variable.

Common situations: Regex pulled from a properties file or user input that is empty; default config value left unfilled; string concatenation producing an empty pattern.

Related errors

AI-assisted analysis of code4craft/webmagic@67816a19d6 (2026-09-08). Data as JSON: /api/errors/4d1eebb42a6a8a51. Report an issue: GitHub.

Appendix: source

Thrown at webmagic-core/src/main/java/us/codecraft/webmagic/selector/RegexSelector.java:32

 * @author code4crafter@gmail.com <br>
 * @since 0.1.0
 */
public class RegexSelector implements Selector {

    private String regexStr;

    private Pattern regex;

    private int group = 1;

    public RegexSelector(String regexStr, int group) {
        this.compileRegex(regexStr);
        this.group = group;
    }

    private void compileRegex(String regexStr) {
        if (StringUtils.isBlank(regexStr)) {
            throw new IllegalArgumentException("regex must not be empty");
        }
        try {
            this.regex = Pattern.compile(regexStr, Pattern.DOTALL | Pattern.CASE_INSENSITIVE);
            this.regexStr = regexStr;
        } catch (PatternSyntaxException e) {
            throw new IllegalArgumentException("invalid regex "+regexStr, e);
        }
    }

    /**
     * Create a RegexSelector. When there is no capture group, the value is set to 0 else set to 1.
     * @param regexStr the regular expression.
     */
    public RegexSelector(String regexStr) {
        this.compileRegex(regexStr);
        if (regex.matcher("").groupCount() == 0) {
            this.group = 0;
        } else {

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