Una guida all'API Java Regular Expressions

1. Panoramica

In questo articolo, discuteremo dell'API Java Regex e di come le espressioni regolari possono essere utilizzate nel linguaggio di programmazione Java.

Nel mondo delle espressioni regolari, ci sono molti gusti diversi tra cui scegliere, come grep, Perl, Python, PHP, awk e molto altro.

Ciò significa che un'espressione regolare che funziona in un linguaggio di programmazione potrebbe non funzionare in un altro. La sintassi delle espressioni regolari in Java è molto simile a quella trovata in Perl.

2. Configurazione

Per utilizzare le espressioni regolari in Java, non è necessaria alcuna configurazione speciale. Il JDK contiene un pacchetto speciale java.util.regex totalmente dedicato alle operazioni regex. Dobbiamo solo importarlo nel nostro codice.

Inoltre, la classe java.lang.String ha anche il supporto regex integrato che usiamo comunemente nel nostro codice.

3. Pacchetto Java Regex

Il pacchetto java.util.regex è costituito da tre classi: Pattern, Matcher e PatternSyntaxException:

  • L' oggetto Pattern è una regex compilata. La classe Pattern non fornisce costruttori pubblici. Per creare un pattern, dobbiamo prima invocare uno dei suoi metodi di compilazione statici pubblici , che poi restituirà un oggetto Pattern . Questi metodi accettano un'espressione regolare come primo argomento.
  • L' oggetto Matcher interpreta il modello ed esegue operazioni di corrispondenza su una stringa di input . Inoltre, non definisce alcun costruttore pubblico. Otteniamo un oggetto Matcher invocando il metodo matcher su un oggetto Pattern .
  • L' oggetto PatternSyntaxException è un'eccezione non controllata che indica un errore di sintassi in un modello di espressione regolare.

Esploreremo queste classi in dettaglio; tuttavia, dobbiamo prima capire come viene costruita una regex in Java.

Se hai già familiarità con regex da un ambiente diverso, potresti trovare alcune differenze, ma sono minime.

4. Semplice esempio

Cominciamo con il caso d'uso più semplice per una regex. Come abbiamo notato in precedenza, quando una regex viene applicata a una stringa, può corrispondere zero o più volte.

La forma più semplice di corrispondenza dei modelli supportata dall'API java.util.regex è la corrispondenza di un valore letterale String . Ad esempio, se l'espressione regolare è foo e la stringa di input è foo , la corrispondenza avrà esito positivo perché le stringhe sono identiche:

@Test public void givenText_whenSimpleRegexMatches_thenCorrect() { Pattern pattern = Pattern.compile("foo"); Matcher matcher = pattern.matcher("foo"); assertTrue(matcher.find()); }

Per prima cosa creiamo un oggetto Pattern chiamando il suo metodo di compilazione statico e passandogli un pattern che vogliamo usare.

Quindi creiamo un oggetto Matcher chiamando il metodo matcher dell'oggetto Pattern e passandogli il testo che vogliamo controllare per le corrispondenze.

Dopodiché, chiamiamo il metodo find nell'oggetto Matcher.

Il metodo find continua ad avanzare nel testo di input e restituisce true per ogni corrispondenza, quindi possiamo usarlo per trovare anche il conteggio delle corrispondenze:

@Test public void givenText_whenSimpleRegexMatchesTwice_thenCorrect() { Pattern pattern = Pattern.compile("foo"); Matcher matcher = pattern.matcher("foofoo"); int matches = 0; while (matcher.find()) { matches++; } assertEquals(matches, 2); }

Poiché eseguiremo più test, possiamo astrarre la logica per trovare il numero di corrispondenze in un metodo chiamato runTest :

public static int runTest(String regex, String text) { Pattern pattern = Pattern.compile(regex); Matcher matcher = pattern.matcher(text); int matches = 0; while (matcher.find()) { matches++; } return matches; }

Quando otteniamo 0 corrispondenze, il test dovrebbe fallire, altrimenti dovrebbe passare.

5. Meta caratteri

I metacaratteri influenzano il modo in cui un modello viene abbinato, in un modo aggiungendo logica al modello di ricerca. L'API Java supporta diversi metacaratteri, il più semplice dei quali è il punto "." che corrisponde a qualsiasi carattere:

@Test public void givenText_whenMatchesWithDotMetach_thenCorrect() { int matches = runTest(".", "foo"); assertTrue(matches > 0); }

Considerando l'esempio precedente in cui regex foo corrispondeva al testo foo e foofoo due volte. Se usassimo il metacarattere punto nella regex, non avremmo due corrispondenze nel secondo caso:

@Test public void givenRepeatedText_whenMatchesOnceWithDotMetach_thenCorrect() { int matches= runTest("foo.", "foofoo"); assertEquals(matches, 1); }

Notare il punto dopo il pippo nella regex. Il matcher trova ogni testo che è preceduto da foo poiché l'ultima parte del punto indica qualsiasi carattere dopo. Quindi, dopo aver trovato il primo foo , il resto viene visto come un personaggio qualsiasi. Ecco perché esiste una sola corrispondenza.

L'API supporta molti altri meta caratteri che esamineremo ulteriormente in questo articolo.

6. Classi di caratteri

Navigando attraverso le specifiche ufficiali della classe Pattern , scopriremo dei riassunti dei costrutti regex supportati. Sotto le classi di personaggi, abbiamo circa 6 costrutti.

6.1. OR Class

Costruito come [abc] . Qualsiasi elemento nel set è abbinato:

@Test public void givenORSet_whenMatchesAny_thenCorrect() { int matches = runTest("[abc]", "b"); assertEquals(matches, 1); }

Se compaiono tutti nel testo, ciascuno viene abbinato separatamente senza riguardo all'ordine:

@Test public void givenORSet_whenMatchesAnyAndAll_thenCorrect() { int matches = runTest("[abc]", "cab"); assertEquals(matches, 3); }

Possono anche essere alternati come parte di una stringa . Nell'esempio seguente, quando creiamo parole diverse alternando la prima lettera con ogni elemento del set, vengono tutte abbinate:

@Test public void givenORSet_whenMatchesAllCombinations_thenCorrect() { int matches = runTest("[bcr]at", "bat cat rat"); assertEquals(matches, 3); }

6.2. NOR Class

Il set precedente viene negato aggiungendo un accento circonflesso come primo elemento:

@Test public void givenNORSet_whenMatchesNon_thenCorrect() { int matches = runTest("[^abc]", "g"); assertTrue(matches > 0); }

Un altro caso:

@Test public void givenNORSet_whenMatchesAllExceptElements_thenCorrect() { int matches = runTest("[^bcr]at", "sat mat eat"); assertTrue(matches > 0); }

6.3. Classe di portata

Possiamo definire una classe che specifica un intervallo entro il quale il testo corrispondente deve rientrare utilizzando un trattino (-), allo stesso modo, possiamo anche negare un intervallo.

Lettere maiuscole corrispondenti:

@Test public void givenUpperCaseRange_whenMatchesUpperCase_ thenCorrect() { int matches = runTest( "[A-Z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 2); }

Lettere minuscole corrispondenti:

@Test public void givenLowerCaseRange_whenMatchesLowerCase_ thenCorrect() { int matches = runTest( "[a-z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 26); }

Corrispondenza di lettere maiuscole e minuscole:

@Test public void givenBothLowerAndUpperCaseRange_ whenMatchesAllLetters_thenCorrect() { int matches = runTest( "[a-zA-Z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 28); }

Corrispondenza di un determinato intervallo di numeri:

@Test public void givenNumberRange_whenMatchesAccurately_ thenCorrect() { int matches = runTest( "[1-5]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 2); }

Corrispondenza di un altro intervallo di numeri:

@Test public void givenNumberRange_whenMatchesAccurately_ thenCorrect2(){ int matches = runTest( "[30-35]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 1); }

6.4. Classe dell'Unione

Una classe di caratteri union è il risultato della combinazione di due o più classi di caratteri:

@Test public void givenTwoSets_whenMatchesUnion_thenCorrect() { int matches = runTest("[1-3[7-9]]", "123456789"); assertEquals(matches, 6); }

Il test precedente corrisponderà solo a 6 dei 9 numeri interi perché il set di unione salta 4, 5 e 6.

6.5. Classe di intersezione

Simile alla classe union, questa classe risulta dalla selezione di elementi comuni tra due o più insiemi. Per applicare l'intersezione, usiamo && :

@Test public void givenTwoSets_whenMatchesIntersection_thenCorrect() { int matches = runTest("[1-6&&[3-9]]", "123456789"); assertEquals(matches, 4); }

We get 4 matches because the intersection of the two sets has only 4 elements.

6.6. Subtraction Class

We can use subtraction to negate one or more character classes, for example matching a set of odd decimal numbers:

@Test public void givenSetWithSubtraction_whenMatchesAccurately_thenCorrect() { int matches = runTest("[0-9&&[^2468]]", "123456789"); assertEquals(matches, 5); }

Only 1,3,5,7,9 will be matched.

7. Predefined Character Classes

The Java regex API also accepts predefined character classes. Some of the above character classes can be expressed in shorter form though making the code less intuitive. One special aspect of the Java version of this regex is the escape character.

As we will see, most characters will start with a backslash, which has a special meaning in Java. For these to be compiled by the Pattern class – the leading backslash must be escaped i.e. \d becomes \\d.

Matching digits, equivalent to [0-9]:

@Test public void givenDigits_whenMatches_thenCorrect() { int matches = runTest("\\d", "123"); assertEquals(matches, 3); }

Matching non-digits, equivalent to [^0-9]:

@Test public void givenNonDigits_whenMatches_thenCorrect() { int mathces = runTest("\\D", "a6c"); assertEquals(matches, 2); }

Matching white space:

@Test public void givenWhiteSpace_whenMatches_thenCorrect() { int matches = runTest("\\s", "a c"); assertEquals(matches, 1); }

Matching non-white space:

@Test public void givenNonWhiteSpace_whenMatches_thenCorrect() { int matches = runTest("\\S", "a c"); assertEquals(matches, 2); }

Matching a word character, equivalent to [a-zA-Z_0-9]:

@Test public void givenWordCharacter_whenMatches_thenCorrect() { int matches = runTest("\\w", "hi!"); assertEquals(matches, 2); }

Matching a non-word character:

@Test public void givenNonWordCharacter_whenMatches_thenCorrect() { int matches = runTest("\\W", "hi!"); assertEquals(matches, 1); }

8. Quantifiers

The Java regex API also allows us to use quantifiers. These enable us to further tweak the match's behavior by specifying the number of occurrences to match against.

To match a text zero or one time, we use the ? quantifier:

@Test public void givenZeroOrOneQuantifier_whenMatches_thenCorrect() { int matches = runTest("\\a?", "hi"); assertEquals(matches, 3); }

Alternatively, we can use the brace syntax, also supported by the Java regex API:

@Test public void givenZeroOrOneQuantifier_whenMatches_thenCorrect2() { int matches = runTest("\\a{0,1}", "hi"); assertEquals(matches, 3); }

This example introduces the concept of zero-length matches. It so happens that if a quantifier's threshold for matching is zero, it always matches everything in the text including an empty String at the end of every input. This means that even if the input is empty, it will return one zero-length match.

This explains why we get 3 matches in the above example despite having a String of length two. The third match is zero-length empty String.

To match a text zero or limitless times, we us * quantifier, it is just similar to ?:

@Test public void givenZeroOrManyQuantifier_whenMatches_thenCorrect() { int matches = runTest("\\a*", "hi"); assertEquals(matches, 3); }

Supported alternative:

@Test public void givenZeroOrManyQuantifier_whenMatches_thenCorrect2() { int matches = runTest("\\a{0,}", "hi"); assertEquals(matches, 3); }

The quantifier with a difference is +, it has a matching threshold of 1. If the required String does not occur at all, there will be no match, not even a zero-length String:

@Test public void givenOneOrManyQuantifier_whenMatches_thenCorrect() { int matches = runTest("\\a+", "hi"); assertFalse(matches); }

Supported alternative:

@Test public void givenOneOrManyQuantifier_whenMatches_thenCorrect2() { int matches = runTest("\\a{1,}", "hi"); assertFalse(matches); }

As it is in Perl and other languages, the brace syntax can be used to match a given text a number of times:

@Test public void givenBraceQuantifier_whenMatches_thenCorrect() { int matches = runTest("a{3}", "aaaaaa"); assertEquals(matches, 2); }

In the above example, we get two matches since a match occurs only if a appears three times in a row. However, in the next test we won't get a match since the text only appears two times in a row:

@Test public void givenBraceQuantifier_whenFailsToMatch_thenCorrect() { int matches = runTest("a{3}", "aa"); assertFalse(matches > 0); }

When we use a range in the brace, the match will be greedy, matching from the higher end of the range:

@Test public void givenBraceQuantifierWithRange_whenMatches_thenCorrect() { int matches = runTest("a{2,3}", "aaaa"); assertEquals(matches, 1); }

We've specified at least two occurrences but not exceeding three, so we get a single match instead where the matcher sees a single aaa and a lone a which can't be matched.

However, the API allows us to specify a lazy or reluctant approach such that the matcher can start from the lower end of the range in which case matching two occurrences as aa and aa:

@Test public void givenBraceQuantifierWithRange_whenMatchesLazily_thenCorrect() { int matches = runTest("a{2,3}?", "aaaa"); assertEquals(matches, 2); }

9. Capturing Groups

The API also allows us to treat multiple characters as a single unit through capturing groups.

It will attache numbers to the capturing groups and allow back referencing using these numbers.

In this section, we will see a few examples on how to use capturing groups in Java regex API.

Let's use a capturing group that matches only when an input text contains two digits next to each other:

@Test public void givenCapturingGroup_whenMatches_thenCorrect() { int maches = runTest("(\\d\\d)", "12"); assertEquals(matches, 1); }

The number attached to the above match is 1, using a back reference to tell the matcher that we want to match another occurrence of the matched portion of the text. This way, instead of:

@Test public void givenCapturingGroup_whenMatches_thenCorrect2() { int matches = runTest("(\\d\\d)", "1212"); assertEquals(matches, 2); }

Where there are two separate matches for the input, we can have one match but propagating the same regex match to span the entire length of the input using back referencing:

@Test public void givenCapturingGroup_whenMatchesWithBackReference_ thenCorrect() { int matches = runTest("(\\d\\d)\\1", "1212"); assertEquals(matches, 1); }

Where we would have to repeat the regex without back referencing to achieve the same result:

@Test public void givenCapturingGroup_whenMatches_thenCorrect3() { int matches = runTest("(\\d\\d)(\\d\\d)", "1212"); assertEquals(matches, 1); }

Similarly, for any other number of repetitions, back referencing can make the matcher see the input as a single match:

@Test public void givenCapturingGroup_whenMatchesWithBackReference_ thenCorrect2() { int matches = runTest("(\\d\\d)\\1\\1\\1", "12121212"); assertEquals(matches, 1); }

But if you change even the last digit, the match will fail:

@Test public void givenCapturingGroupAndWrongInput_ whenMatchFailsWithBackReference_thenCorrect() { int matches = runTest("(\\d\\d)\\1", "1213"); assertFalse(matches > 0); }

It is important not to forget the escape backslashes, this is crucial in Java syntax.

10. Boundary Matchers

The Java regex API also supports boundary matching. If we care about where exactly in the input text the match should occur, then this is what we are looking for. With the previous examples, all we cared about was whether a match was found or not.

To match only when the required regex is true at the beginning of the text, we use the caret ^.

This test will fail since the text dog can be found at the beginning:

@Test public void givenText_whenMatchesAtBeginning_thenCorrect() { int matches = runTest("^dog", "dogs are friendly"); assertTrue(matches > 0); }

The following test will fail:

@Test public void givenTextAndWrongInput_whenMatchFailsAtBeginning_ thenCorrect() { int matches = runTest("^dog", "are dogs are friendly?"); assertFalse(matches > 0); }

To match only when the required regex is true at the end of the text, we use the dollar character $. A match will be found in the following case:

@Test public void givenText_whenMatchesAtEnd_thenCorrect() { int matches = runTest("dog$", "Man's best friend is a dog"); assertTrue(matches > 0); }

And no match will be found here:

@Test public void givenTextAndWrongInput_whenMatchFailsAtEnd_thenCorrect() { int matches = runTest("dog$", "is a dog man's best friend?"); assertFalse(matches > 0); }

If we want a match only when the required text is found at a word boundary, we use \\b regex at the beginning and end of the regex:

Space is a word boundary:

@Test public void givenText_whenMatchesAtWordBoundary_thenCorrect() { int matches = runTest("\\bdog\\b", "a dog is friendly"); assertTrue(matches > 0); }

The empty string at the beginning of a line is also a word boundary:

@Test public void givenText_whenMatchesAtWordBoundary_thenCorrect2() { int matches = runTest("\\bdog\\b", "dog is man's best friend"); assertTrue(matches > 0); }

These tests pass because the beginning of a String, as well as space between one text and another, marks a word boundary, however, the following test shows the opposite:

@Test public void givenWrongText_whenMatchFailsAtWordBoundary_thenCorrect() { int matches = runTest("\\bdog\\b", "snoop dogg is a rapper"); assertFalse(matches > 0); }

Two-word characters appearing in a row does not mark a word boundary, but we can make it pass by changing the end of the regex to look for a non-word boundary:

@Test public void givenText_whenMatchesAtWordAndNonBoundary_thenCorrect() { int matches = runTest("\\bdog\\B", "snoop dogg is a rapper"); assertTrue(matches > 0); }

11. Pattern Class Methods

Previously, we have only created Pattern objects in a basic way. However, this class has another variant of the compile method that accepts a set of flags alongside the regex argument affecting the way the pattern is matched.

These flags are simply abstracted integer values. Let's overload the runTest method in the test class so that it can take a flag as the third argument:

public static int runTest(String regex, String text, int flags) { pattern = Pattern.compile(regex, flags); matcher = pattern.matcher(text); int matches = 0; while (matcher.find()){ matches++; } return matches; }

In this section, we will look at the different supported flags and how they are used.

Pattern.CANON_EQ

This flag enables canonical equivalence. When specified, two characters will be considered to match if, and only if, their full canonical decompositions match.

Consider the accented Unicode character é. Its composite code point is u00E9. However, Unicode also has a separate code point for its component characters e, u0065 and the acute accent, u0301. In this case, composite character u00E9 is indistinguishable from the two character sequence u0065 u0301.

By default, matching does not take canonical equivalence into account:

@Test public void givenRegexWithoutCanonEq_whenMatchFailsOnEquivalentUnicode_thenCorrect() { int matches = runTest("\u00E9", "\u0065\u0301"); assertFalse(matches > 0); }

But if we add the flag, then the test will pass:

@Test public void givenRegexWithCanonEq_whenMatchesOnEquivalentUnicode_thenCorrect() { int matches = runTest("\u00E9", "\u0065\u0301", Pattern.CANON_EQ); assertTrue(matches > 0); }

Pattern.CASE_INSENSITIVE

This flag enables matching regardless of case. By default matching takes case into account:

@Test public void givenRegexWithDefaultMatcher_whenMatchFailsOnDifferentCases_thenCorrect() { int matches = runTest("dog", "This is a Dog"); assertFalse(matches > 0); }

So using this flag, we can change the default behavior:

@Test public void givenRegexWithCaseInsensitiveMatcher _whenMatchesOnDifferentCases_thenCorrect() { int matches = runTest( "dog", "This is a Dog", Pattern.CASE_INSENSITIVE); assertTrue(matches > 0); }

We can also use the equivalent, embedded flag expression to achieve the same result:

@Test public void givenRegexWithEmbeddedCaseInsensitiveMatcher _whenMatchesOnDifferentCases_thenCorrect() { int matches = runTest("(?i)dog", "This is a Dog"); assertTrue(matches > 0); }

Pattern.COMMENTS

The Java API allows one to include comments using # in the regex. This can help in documenting complex regex that may not be immediately obvious to another programmer.

The comments flag makes the matcher ignore any white space or comments in the regex and only consider the pattern. In the default matching mode the following test would fail:

@Test public void givenRegexWithComments_whenMatchFailsWithoutFlag_thenCorrect() { int matches = runTest( "dog$ #check for word dog at end of text", "This is a dog"); assertFalse(matches > 0); }

This is because the matcher will look for the entire regex in the input text, including the spaces and the # character. But when we use the flag, it will ignore the extra spaces and the every text starting with # will be seen as a comment to be ignored for each line:

@Test public void givenRegexWithComments_whenMatchesWithFlag_thenCorrect() { int matches = runTest( "dog$ #check end of text","This is a dog", Pattern.COMMENTS); assertTrue(matches > 0); }

There is also an alternative embedded flag expression for this:

@Test public void givenRegexWithComments_whenMatchesWithEmbeddedFlag_thenCorrect() { int matches = runTest( "(?x)dog$ #check end of text", "This is a dog"); assertTrue(matches > 0); }

Pattern.DOTALL

By default, when we use the dot “.” expression in regex, we are matching every character in the input String until we encounter a new line character.

Using this flag, the match will include the line terminator as well. We will understand better with the following examples. These examples will be a little different. Since we are interested in asserting against the matched String, we will use matcher‘s group method which returns the previous match.

First, we will see the default behavior:

@Test public void givenRegexWithLineTerminator_whenMatchFails_thenCorrect() { Pattern pattern = Pattern.compile("(.*)"); Matcher matcher = pattern.matcher( "this is a text" + System.getProperty("line.separator") + " continued on another line"); matcher.find(); assertEquals("this is a text", matcher.group(1)); }

As we can see, only the first part of the input before the line terminator is matched.

Now in dotall mode, the entire text including the line terminator will be matched:

@Test public void givenRegexWithLineTerminator_whenMatchesWithDotall_thenCorrect() { Pattern pattern = Pattern.compile("(.*)", Pattern.DOTALL); Matcher matcher = pattern.matcher( "this is a text" + System.getProperty("line.separator") + " continued on another line"); matcher.find(); assertEquals( "this is a text" + System.getProperty("line.separator") + " continued on another line", matcher.group(1)); }

We can also use an embedded flag expression to enable dotall mode:

@Test public void givenRegexWithLineTerminator_whenMatchesWithEmbeddedDotall _thenCorrect() { Pattern pattern = Pattern.compile("(?s)(.*)"); Matcher matcher = pattern.matcher( "this is a text" + System.getProperty("line.separator") + " continued on another line"); matcher.find(); assertEquals( "this is a text" + System.getProperty("line.separator") + " continued on another line", matcher.group(1)); }

Pattern.LITERAL

When in this mode, matcher gives no special meaning to any metacharacters, escape characters or regex syntax. Without this flag, the matcher will match the following regex against any input String:

@Test public void givenRegex_whenMatchesWithoutLiteralFlag_thenCorrect() { int matches = runTest("(.*)", "text"); assertTrue(matches > 0); }

This is the default behavior we have been seeing in all the examples. However, with this flag, no match will be found, since the matcher will be looking for (.*) instead of interpreting it:

@Test public void givenRegex_whenMatchFailsWithLiteralFlag_thenCorrect() { int matches = runTest("(.*)", "text", Pattern.LITERAL); assertFalse(matches > 0); }

Now if we add the required string, the test will pass:

@Test public void givenRegex_whenMatchesWithLiteralFlag_thenCorrect() { int matches = runTest("(.*)", "text(.*)", Pattern.LITERAL); assertTrue(matches > 0); }

There is no embedded flag character for enabling literal parsing.

Pattern.MULTILINE

By default ^ and $ metacharacters match absolutely at the beginning and at the end respectively of the entire input String. The matcher disregards any line terminators:

@Test public void givenRegex_whenMatchFailsWithoutMultilineFlag_thenCorrect() { int matches = runTest( "dog$", "This is a dog" + System.getProperty("line.separator") + "this is a fox"); assertFalse(matches > 0); }

The match fails because the matcher searches for dog at the end of the entire String but the dog is present at the end of the first line of the string.

However, with the flag, the same test will pass since the matcher now takes into account line terminators. So the String dog is found just before the line terminates, hence success:

@Test public void givenRegex_whenMatchesWithMultilineFlag_thenCorrect() { int matches = runTest( "dog$", "This is a dog" + System.getProperty("line.separator") + "this is a fox", Pattern.MULTILINE); assertTrue(matches > 0); }

Here is the embedded flag version:

@Test public void givenRegex_whenMatchesWithEmbeddedMultilineFlag_ thenCorrect() { int matches = runTest( "(?m)dog$", "This is a dog" + System.getProperty("line.separator") + "this is a fox"); assertTrue(matches > 0); }

12. Matcher Class Methods

In this section, we will look at some useful methods of the Matcher class. We will group them according to functionality for clarity.

12.1. Index Methods

Index methods provide useful index values that show precisely where the match was found in the input String . In the following test, we will confirm the start and end indices of the match for dog in the input String :

@Test public void givenMatch_whenGetsIndices_thenCorrect() { Pattern pattern = Pattern.compile("dog"); Matcher matcher = pattern.matcher("This dog is mine"); matcher.find(); assertEquals(5, matcher.start()); assertEquals(8, matcher.end()); }

12.2. Study Methods

Study methods go through the input String and return a boolean indicating whether or not the pattern is found. Commonly used are matches and lookingAt methods.

The matches and lookingAt methods both attempt to match an input sequence against a pattern. The difference, is that matches requires the entire input sequence to be matched, while lookingAt does not.

Both methods start at the beginning of the input String :

@Test public void whenStudyMethodsWork_thenCorrect() { Pattern pattern = Pattern.compile("dog"); Matcher matcher = pattern.matcher("dogs are friendly"); assertTrue(matcher.lookingAt()); assertFalse(matcher.matches()); }

The matches method will return true in a case like so:

@Test public void whenMatchesStudyMethodWorks_thenCorrect() { Pattern pattern = Pattern.compile("dog"); Matcher matcher = pattern.matcher("dog"); assertTrue(matcher.matches()); }

12.3. Replacement Methods

Replacement methods are useful to replace text in an input string. The common ones are replaceFirst and replaceAll.

I metodi replaceFirst e replaceAll sostituiscono il testo che corrisponde a una data espressione regolare. Come indicano i loro nomi, replaceFirst sostituisce la prima occorrenza e replaceAll sostituisce tutte le occorrenze:

@Test public void whenReplaceFirstWorks_thenCorrect() { Pattern pattern = Pattern.compile("dog"); Matcher matcher = pattern.matcher( "dogs are domestic animals, dogs are friendly"); String newStr = matcher.replaceFirst("cat"); assertEquals( "cats are domestic animals, dogs are friendly", newStr); }

Sostituisci tutte le occorrenze:

@Test public void whenReplaceAllWorks_thenCorrect() { Pattern pattern = Pattern.compile("dog"); Matcher matcher = pattern.matcher( "dogs are domestic animals, dogs are friendly"); String newStr = matcher.replaceAll("cat"); assertEquals("cats are domestic animals, cats are friendly", newStr); }

Il metodo replaceAll ci consente di sostituire tutte le corrispondenze con la stessa sostituzione. Se vogliamo sostituire le corrispondenze caso per caso, avremmo bisogno di una tecnica di sostituzione del token.

13. Conclusione

In questo articolo abbiamo imparato come utilizzare le espressioni regolari in Java e abbiamo anche esplorato le caratteristiche più importanti del pacchetto java.util.regex .

Il codice sorgente completo per il progetto, inclusi tutti gli esempi di codice usati qui, può essere trovato nel progetto GitHub.