Is Java Regex Thread Safe?

Solution 1:

Yes, from the Java API documentation for the Pattern class

Instances of this (Pattern) class are immutable and are safe for use by multiple concurrent threads. Instances of the Matcher class are not safe for such use.

If you are looking at performance centric code, attempt to reset the Matcher instance using the reset() method, instead of creating new instances. This would reset the state of the Matcher instance, making it usable for the next regex operation. In fact, it is the state maintained in the Matcher instance that is responsible for it to be unsafe for concurrent access.

Solution 2:

Thread-safety with regular expressions in Java

SUMMARY:

The Java regular expression API has been designed to allow a single compiled pattern to be shared across multiple match operations.

You can safely call Pattern.matcher() on the same pattern from different threads and safely use the matchers concurrently. Pattern.matcher() is safe to construct matchers without synchronization. Although the method isn't synchronized, internal to the Pattern class, a volatile variable called compiled is always set after constructing a pattern and read at the start of the call to matcher(). This forces any thread referring to the Pattern to correctly "see" the contents of that object.

On the other hand, you shouldn't share a Matcher between different threads. Or at least, if you ever did, you should use explicit synchronization.