PatternSyntaxException

Question

Following String causes PatternSyntaxException:

Pattern.compile("*\\.*");

I want to create a pattern so that I can filter all files with the name in the following form: "*.*"

How can I do that?

Answer 1

To match all strings with a . in the name, you do:

Pattern.compile(".*[.].*");

To break it down:

• .* match any number of arbitrary character
• [.] match a dot. (yes, \\. works too)
• .* match any number of arbitrary character

Demo:

Pattern p = Pattern.compile(".*[.].*");

System.out.println(p.matcher("hello.txt").matches()); // true
System.out.println(p.matcher("hellotxt").matches());  // false

Note that the string with just one dot, "." matches as well. To ensure that you have some characters in front and after the dot, you could change the * to +: .+[.].+.

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The reason you get PatternSyntaxException:

The * operator is to be interpreted as "the previous character repeated zero or more times". Since you started your expression with * there was no character to repeat, thus an exception was thrown.

Answer 2

The * character has a different meaning in regular expressions than when used on the command line as a file wildcard. More information about the Java Pattern regular expression syntax can be found here: http://docs.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html

This will find the text you are looking to match:

Pattern.compile(".*\\..*");