Escapes regex characters in a string relatively fast.

RegexEscaping.java

package io.github.sauranbone.plang;

import org.apache.commons.lang3.StringUtils;

import java.util.regex.Pattern;

/**
 * @author Vinzent Zeband
 * @version 06:07 CET, 13.02.2022
 * @since 1.0
 */
public class RegexEscaping {


    /**
     * All special characters of regular expression.
     */
    public static final String REGEX_SPECIAL_CHARACTERS = "<([{\\^-=$!|]})?*+.>";

    /**
     * This is equivalent to commons-lang StringUtils#EMPTY.
     **/
    private static final String EMPTY_STRING = "";

    /**
     * Escapes every character in {@code str} that is one of regular
     * expression special characters and returns the mutated and regex
     * escaped string.
     * <p>If {@code str} is null or empty, an empty string is returned.
     *
     * @param str the string to be escaped
     * @return the escaped string
     * @apiNote Alternative to {@link Pattern#quote(String)}.
     * @see #REGEX_SPECIAL_CHARACTERS
     */
    public static String escapeRegex(String str) {
        if (str == null || str.isEmpty())
            return EMPTY_STRING;
        final int n = str.length();
        final char escp = '\\';
        if (n == 1) {
            //Fastpath regex escaping for single character
            final char cp = str.charAt(0);
            return (isSpecialRegexCharacter(cp)
                    ? String.valueOf(escp) : EMPTY_STRING)
                    + cp;
        }
        StringBuilder builder = new StringBuilder(n);
        for (int i = 0; i < n; i++) {
            char ch = str.charAt(i);
            if (isSpecialRegexCharacter(ch)) {
                //Append escaping to this character
                builder.append(escp);
            }
            builder.append(ch);
        }
        return builder.toString();
    }

    /**
     * Returns true if {@code ch} is one of the regular expressions special
     * characters that are defined in this constant.
     *
     * @param ch the character to test
     * @return false if {@code ch} is not a special character that can be
     * escaped
     * @see #REGEX_SPECIAL_CHARACTERS
     */
    public static boolean isSpecialRegexCharacter(char ch) {
        if (ch < 21 || ch > 0x7d)
            return false;
        return REGEX_SPECIAL_CHARACTERS.indexOf(ch) != -1;
    }

}