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Regular Expressions
Methods of the PatternSyntaxException Class
A
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 =
Pattern.compile(console.readLine("%nEnter your regex: "));
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.
