Panduan Untuk Java Regular Expressions API

1. Ikhtisar

Pada artikel ini, kita akan membahas Java Regex API dan bagaimana ekspresi reguler dapat digunakan dalam bahasa pemrograman Java.

Dalam dunia ekspresi reguler, ada banyak ragam yang dapat dipilih, seperti grep, Perl, Python, PHP, awk, dan banyak lagi.

Ini berarti ekspresi reguler yang berfungsi di satu bahasa pemrograman mungkin tidak berfungsi di bahasa lain. Sintaks ekspresi reguler di Java paling mirip dengan yang ditemukan di Perl.

2. Penyiapan

Untuk menggunakan ekspresi reguler di Java, kita tidak memerlukan pengaturan khusus. JDK berisi paket khusus java.util.regex yang sepenuhnya didedikasikan untuk operasi regex. Kami hanya perlu mengimpornya ke dalam kode kami.

Selain itu, kelas java.lang.String juga memiliki dukungan regex bawaan yang biasa kita gunakan dalam kode kita.

3. Paket Java Regex

The java.util.regex paket terdiri dari tiga kelas: Pola, Matcher dan PatternSyntaxException:

  • Objek pola adalah regex terkompilasi. Kelas Pattern tidak menyediakan konstruktor publik. Untuk membuat pola, pertama-tama kita harus memanggil salah satu metode kompilasi statis publiknya , yang kemudian akan mengembalikan objek Pattern . Metode ini menerima ekspresi reguler sebagai argumen pertama.
  • Objek Matcher menafsirkan pola dan melakukan operasi pencocokan terhadap String masukan . Ini juga tidak mendefinisikan konstruktor publik. Kami mendapatkan objek Matcher dengan menjalankan metode matcher pada objek Pattern .
  • Objek PatternSyntaxException adalah pengecualian yang tidak dicentang yang menunjukkan kesalahan sintaks dalam pola ekspresi reguler.

Kami akan menjelajahi kelas-kelas ini secara rinci; namun, pertama-tama kita harus memahami bagaimana regex dibuat di Java.

Jika Anda sudah terbiasa dengan regex dari lingkungan yang berbeda, Anda mungkin menemukan perbedaan tertentu, tetapi perbedaannya minimal.

4. Contoh Sederhana

Mari kita mulai dengan kasus penggunaan paling sederhana untuk regex. Seperti yang kita catat sebelumnya, ketika ekspresi reguler diterapkan ke String, itu mungkin cocok dengan nol atau lebih kali.

Bentuk paling dasar dari pencocokan pola yang didukung oleh java.util.regex API adalah pencocokan literal String . Misalnya, jika ekspresi reguler adalah foo dan String masukan adalah foo , pencocokan akan berhasil karena String identik:

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

Pertama-tama kita membuat objek Pattern dengan memanggil metode kompilasi statisnya dan meneruskannya ke pola yang ingin kita gunakan.

Kemudian kita membuat objek Matcher dengan memanggil metode matcher objek Pattern dan meneruskan teks yang ingin kita periksa kecocokannya.

Setelah itu, kita memanggil metode temukan di objek Matcher.

The menemukan metode terus maju melalui input teks dan kembali berlaku untuk setiap pertandingan, sehingga kita dapat menggunakannya untuk menemukan jumlah pertandingan juga:

@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); }

Karena kita akan menjalankan lebih banyak tes, kita bisa mengabstraksi logika untuk menemukan jumlah kecocokan dalam metode yang disebut 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; }

Ketika kami mendapatkan 0 pertandingan, tes akan gagal, jika tidak, tes akan lulus.

5. Karakter Meta

Karakter meta memengaruhi cara suatu pola dicocokkan, dengan cara menambahkan logika ke pola pencarian. Java API mendukung beberapa karakter meta, yang paling mudah adalah titik "." yang cocok dengan karakter apa pun:

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

Mempertimbangkan contoh sebelumnya di mana regex foo cocok dengan teks foo dan foofoo dua kali. Jika kita menggunakan karakter metak titik di regex, kita tidak akan mendapatkan dua kecocokan dalam kasus kedua:

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

Perhatikan titik setelah foo di regex. Pencocokan cocok dengan setiap teks yang diawali dengan foo karena bagian titik terakhir berarti karakter setelahnya. Jadi setelah menemukan foo pertama , sisanya dilihat sebagai karakter apa saja. Itulah mengapa hanya ada satu pertandingan.

API mendukung beberapa karakter meta lainnya yang akan kita bahas lebih lanjut di artikel ini.

6. Kelas Karakter

Menjelajahi spesifikasi kelas Pattern resmi , kita akan menemukan ringkasan konstruksi regex yang didukung. Di bawah kelas karakter, kami memiliki sekitar 6 konstruksi.

6.1. ATAU Kelas

Dibangun sebagai [abc] . Salah satu elemen dalam himpunan ini cocok:

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

Jika semuanya muncul dalam teks, masing-masing dicocokkan secara terpisah tanpa memperhatikan urutan:

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

Mereka juga bisa diganti-ganti sebagai bagian dari String . Dalam contoh berikut, saat kita membuat kata yang berbeda dengan mengganti huruf pertama dengan setiap elemen himpunan, semuanya cocok:

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

6.2. Kelas NOR

Himpunan di atas dinegasikan dengan menambahkan tanda sisipan sebagai elemen pertama:

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

Kasus lain:

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

6.3. Kelas Rentang

Kita bisa mendefinisikan kelas yang menentukan rentang di mana teks yang cocok harus menggunakan tanda hubung (-), demikian juga, kita juga bisa meniadakan rentang.

Huruf besar yang cocok:

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

Huruf kecil yang cocok:

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

Mencocokkan huruf besar dan huruf kecil:

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

Mencocokkan rentang angka tertentu:

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

Mencocokkan rentang angka lain:

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

6.4. Kelas Serikat

Kelas karakter gabungan adalah hasil dari menggabungkan dua atau lebih kelas karakter:

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

Tes di atas hanya akan mencocokkan 6 dari 9 bilangan bulat karena kumpulan gabungan melompati 4, 5, dan 6.

6.5. Kelas Persimpangan

Mirip dengan kelas gabungan, kelas ini dihasilkan dari pemilihan elemen umum antara dua atau lebih set. Untuk menerapkan persimpangan, kami menggunakan && :

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

Kami mendapatkan 4 pertandingan karena perpotongan dari dua set hanya memiliki 4 elemen.

6.6. Kelas Pengurangan

Kita dapat menggunakan pengurangan untuk meniadakan satu atau lebih kelas karakter, misalnya mencocokkan satu set angka desimal ganjil:

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

Hanya 1,3,5,7,9 yang akan dicocokkan.

7. Kelas Karakter Standar

Java regex API juga menerima kelas karakter yang telah ditentukan sebelumnya. Beberapa kelas karakter di atas dapat diekspresikan dalam bentuk yang lebih pendek meskipun membuat kodenya kurang intuitif. Salah satu aspek khusus dari versi Java regex ini adalah karakter escape.

Seperti yang akan kita lihat, sebagian besar karakter akan dimulai dengan garis miring terbalik, yang memiliki arti khusus di Jawa. Untuk ini akan dikompilasi oleh kelas Pola - garis miring terbalik di depan harus di-escape yaitu \ d menjadi \\ 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.

Metode replaceFirst dan replaceAll menggantikan teks yang cocok dengan ekspresi reguler yang diberikan. Seperti yang ditunjukkan oleh namanya, replaceFirst menggantikan kemunculan pertama, dan replaceAll menggantikan semua kemunculan:

@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); }

Ganti semua kejadian:

@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); }

The replaceAll metode memungkinkan kita untuk mengganti semua pertandingan dengan penggantian yang sama. Jika kami ingin mengganti kecocokan berdasarkan kasus per, kami memerlukan teknik penggantian token.

13. Kesimpulan

Pada artikel ini, kita telah mempelajari bagaimana menggunakan ekspresi reguler di Java dan juga mempelajari fitur-fitur terpenting dari paket java.util.regex .

Kode sumber lengkap untuk proyek termasuk semua contoh kode yang digunakan di sini dapat ditemukan di proyek GitHub.