adds single quote string literal

docs/concepts/datatypes.md

@@ -64,6 +64,10 @@ Any instance of _java.lang.CharSequence_ or _java.lang.Character_ will automatic
 a _STRING_ datatype. Conversion will be done by invoking the _toString()_ method on the input
 object.
 
+By default, the string literal delimiter is the double quote character ("). You can also use both
+`"` and `'` as string literal delimiters by changing the configuration. See
+chapter [Configuration](../configuration/configuration.html) for details.
+
 ### DATE_TIME
 
 Any instance of _java.time.Instant_, _java.time.LocalDate_, _java.time.LocalDateTime_, _java.time.ZoneDateTime_,

docs/configuration/configuration.md

@@ -25,6 +25,7 @@ ExpressionConfiguration configuration=ExpressionConfiguration.builder()
         .powerOfPrecedence(OperatorIfc.OPERATOR_PRECEDENCE_POWER)
         .stripTrailingZeros(true)
         .structuresAllowed(true)
+        .singleQuoteStringLiteralsAllowed(false)
         .build();
 
         Expression expression=new Expression("2.128 + a",configuration);
@@ -88,7 +89,7 @@ See the reference chapter for a list: [Default Constants](../references/constant
 ### Evaluation Value Converter
 
 The converter to use when converting different data types to an _EvaluationValue_.
-The _DefaultEvaluationValueConverter_ is used by default. 
+The _DefaultEvaluationValueConverter_ is used by default.
 
 ### Function Dictionary
 
@@ -163,9 +164,14 @@ no operator or function is defined for this character.
 ### Zone Id
 
 The time zone id. By default, the system default zone ID is used.
+
 ```java
 ExpressionConfiguration configuration=ExpressionConfiguration.builder()
         .zoneId(ZoneId.of("Europe/Berlin"))
         .build();
 ```
 
+### Single Quote String Literals
+
+Specifies if the single quote character (') can be used as a string literal delimiter, not only the
+double quote character (") (default is false). If set to true, the expression will throw a _ParseException_.

src/main/java/com/ezylang/evalex/config/ExpressionConfiguration.java

@@ -222,6 +222,9 @@ public class ExpressionConfiguration {
   /** Support for implicit multiplication, like in (a+b)(b+c) are allowed or not. */
   @Builder.Default @Getter private final boolean implicitMultiplicationAllowed = true;
 
+  /** Support for single quote string literals, like in 'Hello World' are allowed or not. */
+  @Builder.Default @Getter private final boolean singleQuoteStringLiteralsAllowed = false;
+
   /**
    * The power of operator precedence, can be set higher {@link
    * OperatorIfc#OPERATOR_PRECEDENCE_POWER_HIGHER} or to a custom value.

src/main/java/com/ezylang/evalex/parser/Tokenizer.java

@@ -144,7 +144,7 @@ private Token getNextToken() throws ParseException {
     }
 
     // we have a token start, identify and parse it
-    if (currentChar == '"') {
+    if (isAtStringLiteralStart()) {
       return parseStringLiteral();
     } else if (currentChar == '(') {
       return parseBraceOpen();
@@ -464,6 +464,7 @@ private Token parseIdentifier() throws ParseException {
   }
 
   Token parseStringLiteral() throws ParseException {
+    int startChar = currentChar;
     int tokenStartIndex = currentColumnIndex;
     StringBuilder tokenValue = new StringBuilder();
     // skip starting quote
@@ -473,7 +474,7 @@ Token parseStringLiteral() throws ParseException {
       if (currentChar == '\\') {
         consumeChar();
         tokenValue.append(escapeCharacter(currentChar));
-      } else if (currentChar == '"') {
+      } else if (currentChar == startChar) {
         inQuote = false;
       } else {
         tokenValue.append((char) currentChar);
@@ -584,6 +585,11 @@ private boolean isAtIdentifierChar() {
     return Character.isLetter(currentChar) || Character.isDigit(currentChar) || currentChar == '_';
   }
 
+  private boolean isAtStringLiteralStart() {
+    return currentChar == '"'
+        || currentChar == '\'' && configuration.isSingleQuoteStringLiteralsAllowed();
+  }
+
   private void skipBlanks() {
     if (currentChar == -2) {
       // consume first character of expression

src/test/java/com/ezylang/evalex/config/ExpressionConfigurationTest.java

@@ -54,6 +54,7 @@ void testDefaultSetup() {
         .isEqualTo(ExpressionConfiguration.DECIMAL_PLACES_ROUNDING_UNLIMITED);
     assertThat(configuration.isStripTrailingZeros()).isTrue();
     assertThat(configuration.isAllowOverwriteConstants()).isTrue();
+    assertThat(configuration.isSingleQuoteStringLiteralsAllowed()).isFalse();
   }
 
   @Test
@@ -158,6 +159,14 @@ void testStructuresAllowed() {
     assertThat(configuration.isStructuresAllowed()).isFalse();
   }
 
+  @Test
+  void testSingleQuoteStringLiteralsAllowed() {
+    ExpressionConfiguration configuration =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertThat(configuration.isSingleQuoteStringLiteralsAllowed()).isTrue();
+  }
+
   @Test
   void testImplicitMultiplicationAllowed() {
     ExpressionConfiguration configuration =

src/test/java/com/ezylang/evalex/parser/BaseParserTest.java

@@ -28,6 +28,12 @@ public abstract class BaseParserTest {
       TestConfigurationProvider.StandardConfigurationWithAdditionalTestOperators;
 
   void assertAllTokensParsedCorrectly(String input, Token... expectedTokens) throws ParseException {
+    assertAllTokensParsedCorrectly(input, configuration, expectedTokens);
+  }
+
+  void assertAllTokensParsedCorrectly(
+      String input, ExpressionConfiguration configuration, Token... expectedTokens)
+      throws ParseException {
     List<Token> tokensParsed = new Tokenizer(input, configuration).parse();
 
     assertThat(tokensParsed).containsExactly(expectedTokens);

src/test/java/com/ezylang/evalex/parser/TokenizerStringLiteralTest.java

@@ -18,6 +18,7 @@
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 import com.ezylang.evalex.Expression;
+import com.ezylang.evalex.config.ExpressionConfiguration;
 import com.ezylang.evalex.parser.Token.TokenType;
 import org.junit.jupiter.api.Test;
 
@@ -96,4 +97,63 @@ void testErrorUnmatchedQuoteOffset() {
         .isInstanceOf(ParseException.class)
         .hasMessage("Closing quote not found");
   }
+
+  @Test
+  void testSingleQuoteAllowed() {
+    assertThatThrownBy(() -> new Tokenizer("'hello'", configuration).parse())
+        .isInstanceOf(ParseException.class)
+        .hasMessage("Undefined operator '''");
+  }
+
+  @Test
+  void testSingleQuoteOperation() throws ParseException {
+    ExpressionConfiguration config =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertAllTokensParsedCorrectly(
+        "'\"Hello\", ' + \"'World'\"",
+        config,
+        new Token(1, "\"Hello\", ", TokenType.STRING_LITERAL),
+        new Token(13, "+", TokenType.INFIX_OPERATOR),
+        new Token(15, "'World'", TokenType.STRING_LITERAL));
+  }
+
+  @Test
+  void testErrorUnmatchedSingleQuoteStart() {
+    ExpressionConfiguration config =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertThatThrownBy(() -> new Tokenizer("'hello", config).parse())
+        .isInstanceOf(ParseException.class)
+        .hasMessage("Closing quote not found");
+  }
+
+  @Test
+  void testErrorUnmatchedSingleQuoteOffset() {
+    ExpressionConfiguration config =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertThatThrownBy(() -> new Tokenizer("test 'hello", config).parse())
+        .isInstanceOf(ParseException.class)
+        .hasMessage("Closing quote not found");
+  }
+
+  @Test
+  void testErrorUnmatchedDelimiters() {
+    ExpressionConfiguration config =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertThatThrownBy(() -> new Tokenizer("'test\"", config).parse())
+        .isInstanceOf(ParseException.class)
+        .hasMessage("Closing quote not found");
+  }
+
+  @Test
+  void testEscapeSingleQuoteCharacter() throws ParseException {
+    ExpressionConfiguration config =
+        ExpressionConfiguration.builder().singleQuoteStringLiteralsAllowed(true).build();
+
+    assertAllTokensParsedCorrectly(
+        "' \\' \\' \\' '", config, new Token(1, " ' ' ' ", TokenType.STRING_LITERAL));
+  }
 }