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 +: .+[.].+.

---

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(".*\\..*");

Answer 3

Maybe you meant:

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

If not, and asterisk means 'any character' then

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

Answer 4

You have to write:

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

Because * has a special meaning (zero or more) in a regex. So you have to escape it with a \\.

Another way could be:

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

because * loses its other significance when it appears between [ and ].

If you want to parse a filename of format *.*, this should be enough:

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

To match any filename (which may or may not have an extension) you can use

Pattern.compile(".+(\\..+)?");

Answer 5

If you mean, you wand to filter out files with a dot "." in its name, use this one:

Pattern.compile("[^\\.]*\\.[^\\.]*")

Answer 6

Pattern.compile(".*\\/.*\\..*"); use this pattern.It will match all file names which are containing a dot. Asterisk is a special character. It means that the symbol coming before * can be 0 or more times. So you can't write it without character before.
For example for a file name C:/folder.1/folder.2/file.txt pattern will work so:
.* - C:/folder.1/folder.2 (everything until finds the last / character)
\/ - / character
.* - file (everything until finds the last . character (dot))
\\. - . (dot)
.* - txt (everything after . (actually the file extention))