Any decent text editor has a search&replace facility that supports regular expressions.
If however, you have reason to reinvent the wheel in Java, you can do:
Path path = Paths.get("test.txt");
Charset charset = StandardCharsets.UTF_8;
String content = new String(Files.readAllBytes(path), charset);
content = content.replaceAll("foo", "bar");
Files.write(path, content.getBytes(charset));
This only works for Java 7 or newer. If you are stuck on an older Java, you can do:
String content = IOUtils.toString(new FileInputStream(myfile), myencoding);
content = content.replaceAll(myPattern, myReplacement);
IOUtils.write(content, new FileOutputStream(myfile), myencoding);
In this case, you’ll need to add error handling and close the streams after you are done with them.
IOUtils
is documented at http://commons.apache.org/proper/commons-io/javadocs/api-release/org/apache/commons/io/IOUtils.html