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Many classes allow inclusion of escape sequences in character and string literals; examples include java.util.regex.Pattern as well as classes that support XML- and SQL-based actions by passing string arguments to methods. According to the Java Language Specification (JLS), §3.10.6, "Escape Sequences for Character and String Literals" [JLS 20112013],

The character and string escape sequences allow for the representation of some nongraphic characters as well as the single quote, double quote, and backslash characters in character literals (§3.10.4) and string literals (§3.10.5).

Correct use of escape sequences in string literals requires understanding of how the escape sequences are interpreted by the Java compiler . For example, as well as how they are interpreted by any subsequent processor, such as a SQL engine. SQL statements may require escape sequences (for example, sequences containing \t, \n, \r) in certain cases, such as when storing raw text in a database. When representing SQL statements in Java string literals, each escape sequence must be preceded by an extra backslash for correct interpretation.

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In general, for a particular escape character of the form \X, the equivalent Java representation is

\\X

Noncompliant Code Example (String Literal)

This noncompliant code example defines a method, splitWords(), that finds matches between the string literal (WORDS) and the input sequence. It is expected that WORDS would hold the escape sequence for matching a word boundary. However, the Java compiler treats the "\b" literal as a Java escape sequence, and the string WORDS silently compiles to a regular expression that checks for a single backspace character.

public class Splitter {
  private final String WORDS = "\b"; // Interpreted as backspace.
  // Fails to split on word boundaries
  private final String WORDS = "\b";

  public String[] splitWords(String input) {
    Pattern pattern = Pattern.compile(WORDS);
    String[] input_array = pattern.split(input);
    return input_array;
  }
}

Compliant Solution (String Literal)

This compliant solution shows the correctly escaped value of the string literal WORDS that results in a regular expression designed to split on word boundaries:

public class Splitter {
  private final String WORDS = "\\b"; // Interpreted as two chars, '\' and 'b'.
  // Correctly splits on word boundaries
  private final String WORDS = "\\b"; 

  public String[] split(String input){
    Pattern pattern = Pattern.compile(WORDS);
    String[] input_array = pattern.split(input);
    return input_array;
  }
}

Noncompliant Code Example (String Property)

This noncompliant code example uses the same method, splitWords(). This time the WORDS string is loaded from an external properties file.

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In the properties file, the WORD property is once again incorrectly specified as \b. 

WORDS=\b

This is read by the Properties.load() method as a single character b, which causes the split() method to split strings along the letter b. Although the string is interpreted differently than if it were a string literal, as in the previous noncompliant code example, it is still interpreted incorrectly.

WORDS: \b

the interpretation is incorrect.

Compliant Solution (String Property)

This compliant solution shows the correctly escaped value of the WORDS property:

WORDS: =\\b

Applicability

Incorrect use of escape characters in string inputs can result in misinterpretation and potential corruption of data.

Bibliography

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