# Methods of the PatternSyntaxException Class

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PatternSyntaxException is an unchecked exception that indicates a syntax error in a regular expression pattern. The PatternSyntaxException class provides the following methods to help you determine what went wrong:

The following source code, RegexTestHarness2.java, updates our test harness to check for malformed regular expressions:

		import java.io.Console;
import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.util.regex.PatternSyntaxException;

public class RegexTestHarness2 {

public static void main(String[] args){
Pattern pattern = null;
Matcher matcher = null;

Console console = System.console();
if (console == null) {
System.err.println("No console.");
System.exit(1);
}
while (true) {
try{
pattern =

matcher =
pattern.matcher(console.readLine("Enter input string to search: "));
}
catch(PatternSyntaxException pse){
console.format("There is a problem" +
" with the regular expression!%n");
console.format("The pattern in question is: %s%n",
pse.getPattern());
console.format("The description is: %s%n",
pse.getDescription());
console.format("The message is: %s%n",
pse.getMessage());
console.format("The index is: %s%n",
pse.getIndex());
System.exit(0);
}
boolean found = false;
while (matcher.find()) {
console.format("I found the text" +
" \"%s\" starting at " +
"index %d and ending at index %d.%n",
matcher.group(),
matcher.start(),
matcher.end());
found = true;
}
if(!found){
console.format("No match found.%n");
}
}
}
}


To run this test, enter ?i)foo as the regular expression. This mistake is a common scenario in which the programmer has forgotten the opening parenthesis in the embedded flag expression (?i). Doing so will produce the following results:

		Enter your regex: ?i)
There is a problem with the regular expression!
The pattern in question is: ?i)
The description is: Dangling meta character '?'
The message is: Dangling meta character '?' near index 0
?i)
^
The index is: 0


From this output, we can see that the syntax error is a dangling metacharacter (the question mark) at index 0. A missing opening parenthesis is the culprit.

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