1. Información general
En este artículo, discutiremos la API de Java Regex y cómo se pueden usar las expresiones regulares en el lenguaje de programación Java.
En el mundo de las expresiones regulares, hay muchos sabores diferentes para elegir, como grep, Perl, Python, PHP, awk y muchos más.
Esto significa que una expresión regular que funciona en un lenguaje de programación puede no funcionar en otro. La sintaxis de las expresiones regulares en Java es muy similar a la que se encuentra en Perl.
2. Configuración
Para usar expresiones regulares en Java, no necesitamos ninguna configuración especial. El JDK contiene un paquete especial java.util.regex totalmente dedicado a las operaciones de expresiones regulares. Solo necesitamos importarlo a nuestro código.
Además, la clase java.lang.String también tiene soporte para expresiones regulares incorporado que usamos comúnmente en nuestro código.
3. Paquete Java Regex
El paquete java.util.regex consta de tres clases: Pattern, Matcher y PatternSyntaxException:
- El objeto de patrón es una expresión regular compilada. La clase Pattern no proporciona constructores públicos. Para crear un patrón, primero debemos invocar uno de sus métodos públicos de compilación estática , que luego devolverá un objeto Pattern . Estos métodos aceptan una expresión regular como primer argumento.
- El objeto Matcher interpreta el patrón y realiza operaciones de coincidencia con una cadena de entrada . Tampoco define constructores públicos. Obtenemos un objeto Matcher invocando el método matcher en un objeto Pattern .
- El objeto PatternSyntaxException es una excepción sin marcar que indica un error de sintaxis en un patrón de expresión regular.
Exploraremos estas clases en detalle; sin embargo, primero debemos entender cómo se construye una expresión regular en Java.
Si ya está familiarizado con las expresiones regulares de un entorno diferente, puede encontrar ciertas diferencias, pero son mínimas.
4. Ejemplo simple
Comencemos con el caso de uso más simple para una expresión regular. Como señalamos anteriormente, cuando se aplica una expresión regular a una cadena, puede coincidir cero o más veces.
La forma más básica de coincidencia de patrones admitida por la API java.util.regex es la coincidencia de un literal de cadena . Por ejemplo, si la expresión regular es foo y la cadena de entrada es foo , la coincidencia se realizará correctamente porque las cadenas son idénticas:
@Test public void givenText_whenSimpleRegexMatches_thenCorrect() { Pattern pattern = Pattern.compile("foo"); Matcher matcher = pattern.matcher("foo"); assertTrue(matcher.find()); }Primero creamos un objeto Pattern llamando a su método de compilación estático y pasándole un patrón que queremos usar.
Luego creamos un objeto Matcher llamando al método matcher del objeto Pattern y pasándole el texto que queremos verificar en busca de coincidencias.
Después de eso, llamamos al método find en el objeto Matcher.
El método de búsqueda sigue avanzando a través del texto de entrada y devuelve verdadero para cada coincidencia, por lo que también podemos usarlo para encontrar el recuento de coincidencias:
@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); }Dado que ejecutaremos más pruebas, podemos abstraer la lógica para encontrar el número de coincidencias en un método llamado 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; }Cuando obtenemos 0 coincidencias, la prueba debería fallar, de lo contrario, debería pasar.
5. Meta personajes
Los metacaracteres afectan la forma en que se hace coincidir un patrón, de una manera que agrega lógica al patrón de búsqueda. La API de Java admite varios metacaracteres, el más sencillo es el punto "." que coincide con cualquier carácter:
@Test public void givenText_whenMatchesWithDotMetach_thenCorrect() { int matches = runTest(".", "foo"); assertTrue(matches > 0); }Considerando el ejemplo anterior donde regex foo coincidió con el texto foo así como foofoo dos veces. Si usamos el metacarácter de punto en la expresión regular, no obtendríamos dos coincidencias en el segundo caso:
@Test public void givenRepeatedText_whenMatchesOnceWithDotMetach_thenCorrect() { int matches= runTest("foo.", "foofoo"); assertEquals(matches, 1); }Observe el punto después de foo en la expresión regular. El comparador coincide con cada texto que está precedido por foo, ya que la última parte del punto significa cualquier carácter posterior. Entonces, después de encontrar el primer foo , el resto se ve como cualquier personaje. Es por eso que solo hay una coincidencia.
La API admite varios otros metacaracteres que analizaremos más adelante en este artículo.
6. Clases de personajes
Examinando la especificación oficial de la clase Pattern , descubriremos resúmenes de las construcciones de expresiones regulares admitidas. En las clases de personajes, tenemos alrededor de 6 constructos.
6.1. O clase
Construido como [abc] . Cualquiera de los elementos del conjunto coincide:
@Test public void givenORSet_whenMatchesAny_thenCorrect() { int matches = runTest("[abc]", "b"); assertEquals(matches, 1); }Si todos aparecen en el texto, cada uno se empareja por separado sin tener en cuenta el orden:
@Test public void givenORSet_whenMatchesAnyAndAll_thenCorrect() { int matches = runTest("[abc]", "cab"); assertEquals(matches, 3); }También se pueden alternar como parte de una cadena . En el siguiente ejemplo, cuando creamos diferentes palabras alternando la primera letra con cada elemento del conjunto, todas coinciden:
@Test public void givenORSet_whenMatchesAllCombinations_thenCorrect() { int matches = runTest("[bcr]at", "bat cat rat"); assertEquals(matches, 3); }6.2. Clase NOR
El conjunto anterior se niega agregando un símbolo de intercalación como primer elemento:
@Test public void givenNORSet_whenMatchesNon_thenCorrect() { int matches = runTest("[^abc]", "g"); assertTrue(matches > 0); }Otro caso:
@Test public void givenNORSet_whenMatchesAllExceptElements_thenCorrect() { int matches = runTest("[^bcr]at", "sat mat eat"); assertTrue(matches > 0); }6.3. Clase de rango
Podemos definir una clase que especifica un rango dentro del cual el texto coincidente debe caer usando un guión (-), de la misma forma, también podemos negar un rango.
Coincidencia de letras mayúsculas:
@Test public void givenUpperCaseRange_whenMatchesUpperCase_ thenCorrect() { int matches = runTest( "[A-Z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 2); }Coincidencia de letras minúsculas:
@Test public void givenLowerCaseRange_whenMatchesLowerCase_ thenCorrect() { int matches = runTest( "[a-z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 26); }Coincidencia de letras mayúsculas y minúsculas:
@Test public void givenBothLowerAndUpperCaseRange_ whenMatchesAllLetters_thenCorrect() { int matches = runTest( "[a-zA-Z]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 28); }Coincidir con un rango dado de números:
@Test public void givenNumberRange_whenMatchesAccurately_ thenCorrect() { int matches = runTest( "[1-5]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 2); }Coincidencia de otro rango de números:
@Test public void givenNumberRange_whenMatchesAccurately_ thenCorrect2(){ int matches = runTest( "[30-35]", "Two Uppercase alphabets 34 overall"); assertEquals(matches, 1); }6.4. Clase sindical
Una clase de carácter de unión es el resultado de combinar dos o más clases de caracteres:
@Test public void givenTwoSets_whenMatchesUnion_thenCorrect() { int matches = runTest("[1-3[7-9]]", "123456789"); assertEquals(matches, 6); }La prueba anterior solo coincidirá con 6 de los 9 enteros porque el conjunto de unión omite 4, 5 y 6.
6.5. Clase de intersección
Similar a la clase de unión, esta clase resulta de elegir elementos comunes entre dos o más conjuntos. Para aplicar la intersección, usamos && :
@Test public void givenTwoSets_whenMatchesIntersection_thenCorrect() { int matches = runTest("[1-6&&[3-9]]", "123456789"); assertEquals(matches, 4); }Obtenemos 4 coincidencias porque la intersección de los dos conjuntos tiene solo 4 elementos.
6.6. Clase de resta
Podemos usar la resta para negar una o más clases de caracteres, por ejemplo, hacer coincidir un conjunto de números decimales impares:
@Test public void givenSetWithSubtraction_whenMatchesAccurately_thenCorrect() { int matches = runTest("[0-9&&[^2468]]", "123456789"); assertEquals(matches, 5); }Solo se igualarán 1,3,5,7,9 .
7. Clases de caracteres predefinidas
La API de Java regex también acepta clases de caracteres predefinidas. Algunas de las clases de caracteres anteriores se pueden expresar de forma más corta, aunque hacen que el código sea menos intuitivo. Un aspecto especial de la versión Java de esta expresión regular es el carácter de escape.
Como veremos, la mayoría de los caracteres comenzarán con una barra invertida, que tiene un significado especial en Java. Para que estos sean compilados por la clase Pattern , la barra invertida inicial debe escaparse, es decir, \ d se convierte en \\ 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.
Los métodos replaceFirst y replaceAll reemplazan el texto que coincide con una expresión regular determinada. Como lo indican sus nombres, replaceFirst reemplaza la primera aparición y replaceAll reemplaza todas las ocurrencias:
@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); }Reemplazar todas las ocurrencias:
@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); }El método replaceAll nos permite sustituir todas las coincidencias con el mismo reemplazo. Si queremos reemplazar las coincidencias caso por caso, necesitaríamos una técnica de reemplazo de tokens.
13. Conclusión
En este artículo, hemos aprendido a usar expresiones regulares en Java y también hemos explorado las características más importantes del paquete java.util.regex .
El código fuente completo del proyecto, incluidos todos los ejemplos de código utilizados aquí, se puede encontrar en el proyecto GitHub.