14. Regular Expressions

Programming Project 2021/22

14.3. Using Regular Expressions in String Methods

Backslashes with RegExp & Java

The backslash \ is an escape character in Java Strings.

You have to use double backslash \\ in a String to define a single backslash.

If you want to define \w, then you must use "\\w" in your regex.

If you want to use backslash as a literal, you have to type \\\\. This is because \ is also an escape character in regular expressions.

Using regular expressions with String methods

Strings have the following four built-in methods for regular expressions.

matches()
split()
replaceFirst()
replaceAll()

The replace() method we have seen before does NOT support regular expressions.

Using regular expressions with String methods

s.matches(regex):

Evaluates if regex matches s.
Returns true only if the WHOLE string can be matched.

s.split(regex):

Creates an array with substrings of s divided at occurrences of regex.

s.replaceFirst(regex, replacement):

Replaces the first occurrence of regex with replacement.

s.replaceAll(regex, replacement):

Replaces all occurrences of regex with replacement.

Matching string segments

The matches() method checks if the entire string matches the regex.

public class RegexMatch {
 public static void main(String[] args) {
     String phrase = "What, so everyone’s supposed to sleep every single night now? " +
             "You realize that nighttime makes up half of all time?";

     // Does the phrase contain the same two letters in a row?
     boolean matched = phrase.matches(".*(\\w)\\1.*");
     System.out.println("Matched /.*(\\w)\\1.*/: " + matched);

     // Does the phrase equal "night"?
     matched = phrase.matches("night");
     System.out.println("Matched /night/: " + matched);

     // Does the phrase contain the word "night"?
     matched = phrase.matches(".*night.*");
     System.out.println("Matched /.*night.*/: " + matched);
 }
}

Matched /.*(.)\1.*/: true
Matched /night/: false
Matched /.*night.*/: true

Splitting strings using regular expressions

The code below splits the phrase by empty spaces.

Remember that \s is equivalent to [ \r\n\t\f\v ].

public class RegexSplit {

   public static void main(String[] args) {
      String phrase = "What,\tso everyone’s supposed to sleep every single night now?\n" +
              "You realize that nighttime makes up half of all time?";

      String[] words = phrase.split("\\s");
      for (String word : words)
         System.out.println(word);

   }

}

What,
so
everyone’s
supposed
to
sleep
every
single
night
now?
You
realize
that
nighttime
makes
up
half
of
all
time?

Replacing string segments using regular expressions

public class RegexReplace {
   
   public static void main(String[] args) {
      String phrase = "What, so everyone’s supposed to sleep every single night now? " +
				"You realize that nighttime makes up half of all time?";
		
		String replacedFirst = phrase.replaceFirst("\\b\\w{3}\\b", "333");
		System.out.println(replacedFirst);

		String replacedAll = phrase.replaceAll("\\b\\w{3}\\b", "333");
		System.out.println(replacedAll);

		String noPunctuation = phrase.replaceAll("[\\.,'’!?\\-]", "");
		System.out.println(noPunctuation);
	}

}

What, so everyone’s supposed to sleep every single night 333? You realize that nighttime makes up half of all time?
What, so everyone’s supposed to sleep every single night 333? 333 realize that nighttime makes up half of 333 time?
What so everyones supposed to sleep every single night now You realize that nighttime makes up half of all time

Grouping and back reference

You can group parts of your regular expression by using parenthesis: ()

This allows you to assign a repetition operator to a complete group.

(abc){3}

The regex above matches abcabcabc.

Groups create a back reference to part of the regular expression.

A back reference stores the part of the String which matched the group.

It allows you to use this part in the replacement.

Via the dollar sign $ you can refer to a group.

$1 is the first group, $2 the second, etc.

Replacing string segments using back references

public class RegexReplaceGroup {

   public static void main(String[] args) {
      String s = "The windows are open. The apples are green.";
      String s2 = s.replaceAll("(\\w+)s\\s+are", "$1 is");
      System.out.println(s2);
   }

}

The window is open. The apple is green.

Grouping and back reference

You can refer to a group in a regex itself.

The regex below matches repeated letters.

([a-z])\1

If ran against string "My email address is john.doe@unibz.it", the regex matches:

dd
ss

Exercise 1

Write a program that transforms a comma-separated-value string into a semi-colon-separated-value string. Consider the following input.
```
id,name,grade
1,Rick,30
2,Morty,18
3,Summer,24
```
Your code should produce the following output.
```
id;name;grade
1;Rick;30
2;Morty;18
3;Summer;24
```

Now, your output should have quotes for each value between semi-colons.
```
"id";"name";"grade"
"1;"Rick";"30"
"2";"Morty";"18"
"3";"Summer";"24"
```

Exercise 2

Using regular expressions, remove all tags from the HTML string below.

<p>
  <i><b>Rick and Morty</b></i>
  is an American
  <a href="/wiki/Adult_animation" title="Adult animation">adult animated</a>
  <a href="/wiki/Science_fiction" title="Science fiction">science fiction</a>
  <a href="/wiki/Animated_sitcom" title="Animated sitcom">sitcom</a> created by
  <a href="/wiki/Justin_Roiland" title="Justin Roiland">Justin Roiland</a> and
  <a href="/wiki/Dan_Harmon" title="Dan Harmon">Dan Harmon</a> for
  <a href="/wiki/Cartoon_Network" title="Cartoon Network">Cartoon Network</a>'s late-night programming block
  <a href="/wiki/Adult_Swim" title="Adult Swim">Adult Swim</a>.
  The series follows the misadventures of cynical
  <a href="/wiki/Mad_scientist" title="Mad scientist">mad scientist</a>
  <a href="/wiki/Rick_Sanchez_(Rick_and_Morty)" title="Rick Sanchez (Rick and Morty)">Rick Sanchez</a>
  and his good-hearted but fretful grandson
  <a href="/wiki/Morty_Smith" title="Morty Smith">Morty Smith</a>,
  who split their time between domestic life and interdimensional adventures.
</p>

Let's practice writing regular expressions?

Here are some games that can help you practice regular expressions.

References

Part of the material has been taken from the following sources. The usage of the referenced copyrighted work is in line with fair use since it is for nonprofit educational purposes.