Escapes characters that have special meaning to regular expressions : Introduction « Regular Expressions « Java Tutorial

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/*
 * Static String formatting and query routines.
 * Copyright (C) 2001-2005 Stephen Ostermiller
 * http://ostermiller.org/contact.pl?regarding=Java+Utilities
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * See COPYING.TXT for details.
 */


import java.util.HashMap;
import java.util.regex.Pattern;

/**
 * Utilities for String formatting, manipulation, and queries.
 * More information about this class is available from <a target="_top" href=
 * "http://ostermiller.org/utils/StringHelper.html">ostermiller.org</a>.
 *
 * @author Stephen Ostermiller http://ostermiller.org/contact.pl?regarding=Java+Utilities
 * @since ostermillerutils 1.00.00
 */
public class StringHelper {

  /**
   * Escapes characters that have special meaning to
   * regular expressions
   *
   * @param s String to be escaped
   * @return escaped String
   * @throws NullPointerException if s is null.
   *
   * @since ostermillerutils 1.02.25
   */
  public static String escapeRegularExpressionLiteral(String s){
    // According to the documentation in the Pattern class:
    //
    // The backslash character ('\') serves to introduce escaped constructs,
    // as defined in the table above, as well as to quote characters that
    // otherwise would be interpreted as unescaped constructs. Thus the
    // expression \\ matches a single backslash and \{ matches a left brace.
    //
    // It is an error to use a backslash prior to any alphabetic character
    // that does not denote an escaped construct; these are reserved for future
    // extensions to the regular-expression language. A backslash may be used
    // prior to a non-alphabetic character regardless of whether that character
    // is part of an unescaped construct.
    //
    // As a result, escape everything except [0-9a-zA-Z]

    int length = s.length();
    int newLength = length;
    // first check for characters that might
    // be dangerous and calculate a length
    // of the string that has escapes.
    for (int i=0; i<length; i++){
      char c = s.charAt(i);
      if (!((c>='0' && c<='9') || (c>='A' && c<='Z') || (c>='a' && c<='z'))){
        newLength += 1;
      }
    }
    if (length == newLength){
      // nothing to escape in the string
      return s;
    }
    StringBuffer sb = new StringBuffer(newLength);
    for (int i=0; i<length; i++){
      char c = s.charAt(i);
      if (!((c>='0' && c<='9') || (c>='A' && c<='Z') || (c>='a' && c<='z'))){
        sb.append('\\');
      }
      sb.append(c);
    }
    return sb.toString();
  }
}