001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017 package org.apache.commons.jexl2.parser;
018
019 /**
020 * Common constant strings utilities.
021 * <p>
022 * This package methods read JEXL string literals and handle escaping through the
023 * 'backslash' (ie: \) character. Escaping is used to neutralize string delimiters (the single
024 * and double quotes) and read Unicode hexadecimal encoded characters.
025 * </p>
026 * <p>
027 * The only escapable characters are the single and double quotes - ''' and '"' -,
028 * a Unicode sequence starting with 'u' followed by 4 hexadecimals and
029 * the backslash character - '\' - itself.
030 * </p>
031 * <p>
032 * A sequence where '\' occurs before any non-escapable character or sequence has no effect, the
033 * sequence output being the same as the input.
034 * </p>
035 */
036 public class StringParser {
037 /** Default constructor. */
038 public StringParser() {}
039
040 /**
041 * Builds a string, handles escaping through '\' syntax.
042 * @param str the string to build from
043 * @param eatsep whether the separator, the first character, should be considered
044 * @return the built string
045 */
046 public static String buildString(CharSequence str, boolean eatsep) {
047 StringBuilder strb = new StringBuilder(str.length());
048 char sep = eatsep ? str.charAt(0) : 0;
049 int end = str.length() - (eatsep ? 1 : 0);
050 int begin = (eatsep ? 1 : 0);
051 read(strb, str, begin, end, sep);
052 return strb.toString();
053 }
054
055 /**
056 * Read the remainder of a string till a given separator,
057 * handles escaping through '\' syntax.
058 * @param strb the destination buffer to copy characters into
059 * @param str the origin
060 * @param index the offset into the origin
061 * @param sep the separator, single or double quote, marking end of string
062 * @return the offset in origin
063 */
064 public static int readString(StringBuilder strb, CharSequence str, int index, char sep) {
065 return read(strb, str, index, str.length(), sep);
066 }
067
068 /** The length of an escaped unicode sequence. */
069 private static final int UCHAR_LEN = 4;
070
071 /**
072 * Read the remainder of a string till a given separator,
073 * handles escaping through '\' syntax.
074 * @param strb the destination buffer to copy characters into
075 * @param str the origin
076 * @param begin the relative offset in str to begin reading
077 * @param end the relative offset in str to end reading
078 * @param sep the separator, single or double quote, marking end of string
079 * @return the last character offset handled in origin
080 */
081 private static int read(StringBuilder strb, CharSequence str, int begin, int end, char sep) {
082 boolean escape = false;
083 int index = begin;
084 for (; index < end; ++index) {
085 char c = str.charAt(index);
086 if (escape) {
087 if (c == 'u' && (index + UCHAR_LEN) < end && readUnicodeChar(strb, str, index + 1) > 0) {
088 index += UCHAR_LEN;
089 } else {
090 // if c is not an escapable character, re-emmit the backslash before it
091 boolean notSeparator = sep == 0? c != '\'' && c != '"' : c != sep;
092 if (notSeparator && c != '\\') {
093 strb.append('\\');
094 }
095 strb.append(c);
096 }
097 escape = false;
098 continue;
099 }
100 if (c == '\\') {
101 escape = true;
102 continue;
103 }
104 strb.append(c);
105 if (c == sep) {
106 break;
107 }
108 }
109 return index;
110 }
111
112 /** Initial shift value for composing a Unicode char from 4 nibbles (16 - 4). */
113 private static final int SHIFT = 12;
114 /** The base 10 offset used to convert hexa characters to decimal. */
115 private static final int BASE10 = 10;
116 /**
117 * Reads a Unicode escape character.
118 * @param strb the builder to write the character to
119 * @param str the sequence
120 * @param begin the begin offset in sequence (after the '\\u')
121 * @return 0 if char could not be read, 4 otherwise
122 */
123 private static int readUnicodeChar(StringBuilder strb, CharSequence str, int begin) {
124 char xc = 0;
125 int bits = SHIFT;
126 int value = 0;
127 for(int offset = 0; offset < UCHAR_LEN; ++offset) {
128 char c = str.charAt(begin + offset);
129 if (c >= '0' && c <= '9') {
130 value = (c - '0');
131 } else if (c >= 'a' && c <= 'h') {
132 value = (c - 'a' + BASE10);
133 } else if (c >= 'A' && c <= 'H') {
134 value = (c - 'A' + BASE10);
135 } else {
136 return 0;
137 }
138 xc |= value << bits;
139 bits -= UCHAR_LEN;
140 }
141 strb.append(xc);
142 return UCHAR_LEN;
143 }
144
145 /** The last 7bits ascii character. */
146 private static final char LAST_ASCII = 127;
147
148 /**
149 * Escapes a String representation, expand non-ASCII characters as Unicode escape sequence.
150 * @param str the string to escape
151 * @return the escaped representation
152 */
153 public static String escapeString(String str) {
154 if (str == null) {
155 return null;
156 }
157 final int length = str.length();
158 StringBuilder strb = new StringBuilder(length + 2);
159 strb.append('\'');
160 for (int i = 0; i < length; ++i) {
161 char c = str.charAt(i);
162 if (c < LAST_ASCII) {
163 if (c == '\'') {
164 // escape quote
165 strb.append('\\');
166 strb.append('\'');
167 } else if (c == '\\') {
168 // escape backslash
169 strb.append('\\');
170 strb.append('\\');
171 } else {
172 strb.append(c);
173 }
174 } else {
175 // convert to Unicode escape sequence
176 strb.append('\\');
177 strb.append('u');
178 String hex = Integer.toHexString(c);
179 for (int h = hex.length(); h < UCHAR_LEN; ++h) {
180 strb.append('0');
181 }
182 strb.append(hex);
183 }
184 }
185 strb.append('\'');
186 return strb.toString();
187 }
188 }