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regular expression is a sequence of characters that specifies a set of strings and that is used to search, edit, and manipulate text. Like most programming languages Java supports regular expressions. We’ve already learned some basics of the regex language. In this lesson, we’ll explore how it is realized in Java.

Simple matching

First of all, we can create a regular expression by means of a String . Take a look at the following example:

String aleRegex = "ale"; // the "ale" regex


In Java, String data type has built-in support for regular expressions. Strings have a special method called matches that takes a regular expression pattern as its argument and checks whether the string matches this pattern. Keep in mind that the method returns true only when the whole string matches the regexp, otherwise, it returns false. The pattern defined by the regex is applied to the text from left to right.

In the example below, we try to match aleRegex and different strings:

"ale".matches(aleRegex);  // true
"pale".matches(aleRegex); // false, "pale" string has an additional character
"ALE".matches(aleRegex);  // false, uppercase letters don't match lowercase and vice versa


You can see that the string "pale" is not matched by our regex pattern. The reason is that Java regex implementation checks whether the whole string can be fit into the regex pattern, not just some substring. In this regard, Java differs from many other programming languages.

Here is another example. The helloRegex pattern has two words separated by a comma and a whitespace character:

String helloRegex = "Hello, World";

"Hello, World".matches(helloRegex); // true
"Hello, world".matches(helloRegex); // false
"Hello,World".matches(helloRegex); // false


As is evident from the previous examples, when our regex is just a sequence of simple characters, it can be matched only with the exact same string. There are easier ways to compare strings, though, and we wouldn’t really need regular expressions if that was all they could do.

As you remember, the real power of regular expressions lies in special characters that help you define a pattern matching several different strings at once. In this lesson, we will look at two special characters you are already familiar with.

The dot character and the question mark

The dot . matches any single character including letters, digits, spaces, and so on. The only character it is unable to match with is the newline character \n. The examples below should look familiar to you:

String learnRegex = "Learn.Regex";

"Learn Regex".matches(learnRegex); // true
"Learn.Regex".matches(learnRegex); // true
"LearnRegex".matches(learnRegex); // false
"Learn, Regex".matches(learnRegex); // false
"Learn\nRegex".matches(learnRegex); // false


As you remember, the question mark ? is a special character that means “the preceding character or nothing”. Words with slightly different spelling in American and British English serve as a traditional example of this character’s application:

String pattern = "behaviou?r";

"behaviour".matches(pattern); // true
"behavior".matches(pattern);  // true


Now let’s combine the dot character . and the question mark ? in one regex pattern. This combination (.?) basically means “there can be any single character or no character at all”. In the example below, the regex matches any two or one character:

String pattern = "..?";

"I".matches(pattern);  // true
"am".matches(pattern); // true
"".matches(pattern);   // false


Sometimes we want to use the dot character or the question mark, though. Let’s see what we should do about that.

The tricky escape character

Right now you’re probably wondering what should we do if we want to use the dot . or the ? as a regular punctuation mark and not as a special symbol within the regex pattern?

Well, in this case, we should protect our special symbol by putting the backslash \ before it. The backslash is called an escape character because it helps symbols to “escape” their working duties. Note that when you want to use the backslash \ itself in its literal meaning, you need to escape it as well! This way, a double backslash \\ in your regex means a single backslash in the matching string.

However, it gets more complicated when you implement such patterns in your Java program. The backslash \ works as an escape character not only for regular expressions but for String literal as well. So, in fact, we have to use an additional backslash to escape the one we need in the regular expression, just like this:

String endRegex = "The End\\.";

"The End.".matches(endRegex); // true
"The End?".matches(endRegex); // false


For instance, the regular expression for any five-character sequence that ends with a dot looks like this:

String pattern = ".....\\.";

"a1b2c.".matches(pattern); // true
"Wrong.".matches(pattern); // true
"Hello!".matches(pattern); // false


Conclusion

As you can see, regular expressions are a powerful tool for processing strings in Java. They allow us to define common patterns by using regular characters and characters with special meaning, and then check whether strings match these patterns. The key points of this lesson are:

• matches string method is used to compare a regex pattern with a string;
• it returns true only when the whole string matches the regexp;
• when using regexps in Java, we should add extra backslash for escaping symbols, which results in using a double backslash instead of just one.

In the following topics, we will see what other possibilities the Java regexes provide: more special characters, other built-in string methods that take regex patterns, and some advanced classes to process strings.