Java replaceall regex backreference The matching can be case insensitive. Thus, you can make your matcher case-insensitive with a flag: Pattern. With capital cc on the 2000CC. appendTail(StringBuffer). replaceAll(repl). But there is no such group, so there is the exception. group(0)) always captures the whole area that is covered by your regular expression. println(data. Nov 7, 2012 · In case you don't want to use a regex, as a regex implies it is a 'regular' expression. compile(regex, Pattern. Jul 31, 2013 · The first group (m. Sep 13, 2018 · I have the following problem which states. In the official release of Java 7, the constructs to support the named capturing group are: (?<name>capturing text) to define a named group "name" \k<name> to backreference a named group "name" Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand Mar 24, 2012 · java, regular expression, need to escape backslash in regex "There are two interpretations of escape sequences going on: first by the Java compiler, and then by the regexp engine. So, for your case you can do it like, String example = "the sun was shin- ing and the sky bl- ue"; System. *)", "\\2-\\1") // WRONG!!! Mar 19, 2014 · You want to catch the first group that was selected in your regular expression. I used to plant a heel on the first parameter. Even tho such inputs are not impossible. r val REGEX_CLOSED_CURLY_BRACE = """\\}""". Dec 8, 2013 · Your problem is that in a regular expression, the \ character has a special meaning. replaceAll(replacement) then states: The below expression should work correctly to find any number of duplicated words. So you need to escape it as well with a backslash (and a second backslash to make the java compiler happy): String s=" $$"; s = s. I have a long string. Jun 15, 2012 · Although, as you have a double line, you will get a double space. replaceAll("(. You actually quoted the part of the docs that explains this Nov 8, 2014 · Here is the one-liner replaceAll regex solution: since backreferences \1 doesn't have obvious length, Java regex to remove duplicate substrings from string. You actually need to write . *\1 then for the string:"abc me bca" the output is:"abc me bca" can someone explain how is that possible because as per the steps:-[abc] matches a from the input. For example, the string "Abed" shall become "(A)b(e)d". NET, Java, Perl, PCRE, JavaScript, Python, Ruby Jun 7, 2015 · Regex engine have solution for this inefficiency in form of matcher. – This is a block of of text. println(example. Then both \ characters need to be escaped from the Java compiler. Related. Dec 19, 2024 · The key difference between replace() and replaceAll() is that replace() always performs literal String replacement. My regex works fine and groupCount() returns 11. For example: String = "This is a great day, is it not? If there is something, THIS IS it. When you replace with those values, make sure you escape the $ char (if followed with a digit, it will form a backreference and that might lead to an exception, e. Despite the fact that it deviates from the replaceAll name, it does replace all occurrences just like replaceAll(). RegEx Breakup: (?<!^) # If previous position is not line start \\*+ # match 1 or more * (?!$) # If next position is not line end May 23, 2024 · In this tutorial, we'll explore how to use the replaceAll() method in the String class to replace text using regular expressions. In regular expressions, the backslash is also an escape character. 3. several words here are are repeated, and and they should not be. In this case, it's the whole string. Edit i am aware of the fact that backreference ovverride values if the backtraking occurs and the ovverided output will be the new backreference. To understand backreferences, we need to understand group first. \1, \2, \3, etc), but in a replacement string they preceded by a dollar sign (e. Matcher. 정규표현식(Regular expressions), Regex는 문자열에서 어떤 패턴을 찾는데 도움을 줍니다. replaceAll(Pattern. For example, the word "tooth" has the pattern 12213 since both the 't' and 'o' repeat. Java regex intersection with backreference. The literal string "\\" is a single backslash. Is there any way to check for null (if not null then) in the regex? Aug 4, 2017 · See the \b(\w+)\s\1\b regex demo. Feb 9, 2006 · You can use the contents of capturing parentheses in the replacement text via $1, $2, $3, etc. The actual assignment is to implement a Polynomial Processing Java application, and I'm using this to pass polynomial. However, if the group is null, the backreference numbers all get changed. Apr 7, 2017 · The problem is that the backreference values are only honored in the replacement string. This regular expression as a Java string, becomes "\\\\". – Dec 23, 2024 · This method replaces each substring of the string that matches the given regular expression with the given replace_str. java. wav. Regex는 대부분 알고 있지만, 적용할 표현들이 헷갈렸다면 이 글을 참고하시면 좋을 것 같습니다. Can you lend me a hand? P. In this tutorial, we covered how to use back references and lookaround assertions in Java regex for string replacement. Any char can be matched with . but if i take this regex for example: ([abc]+). replaceAll("$2")). ;) (If OP was considering the possibility to get negative lookaheads where there are none (I got that impression for some reason): I don't know of any regex flavors that support possessive quantifiers/atomic groups that do not support negative lookaheads, while there are several with the other way around. regex. The backreference only stores the text value captured, and not the context it was matched in. Unlike referencing a captured group inside a replacement string , a backreference is used inside a regular expression by inlining it's group number preceded by a single backslash. "i think there is a pattern here" does not a regular expression make. If you pretty-print the output, the result looks like (I used the nope element to demonstrate keeping the Apr 19, 2012 · Assuming your string contains the entire word: String resultString = subjectString. Sep 6, 2020 · Use this if you want guaranteed linear performance (O(n+m)) regular expressions in java. So here's my attempt using backreferences. *?(<item>. As is mathematically dictates, re2j doesn't support backreferences and a few other things (see the site for the list), so it cannot run your (. I guess you could then do another replace all to replace double spaces with a single one. If that doesn't work try doing: string. Apr 8, 2011 · str. Now, let’s say we want to replace all dots “. replaceAll("(?<=[^_])[A-Z]","_$0");, just put the negated character class into a positive lookbehind. replaceAll(repl) Therefore, it can be expected the performance between invoking the String. Below is my java code. *", but you want the label treated as plain text (ie not a regex), add regex quotes around the label, either May 31, 2012 · The replace method will replace all occurrences of a char or CharSequence. This is not an anomaly: . $1, $2, $3, and also $0). replaceAll documentation: Dec 19, 2024 · In contrast, replaceAll() uses regex to match patterns, making it a more advanced tool for string manipulation. If file. replaceAll(repl) Using backreferences to preserve case: Nov 8, 2014 · Here is the one-liner replaceAll regex solution: since backreferences \1 doesn't have obvious length, Java regex to remove duplicate substrings from string. If you pretty-print the output, the result looks like (I used the nope element to demonstrate keeping the May 26, 2014 · You can use it as first argument of replaceAll (because replace as mentioned earlier doesn't accept regex). )\\1{1,} expression - that's because mathematically speaking it isn't possible to do that in O(n) time. IllegalArgumentException: character to be escaped is missing Nov 24, 2015 · More details at Java String#replaceAll documentation: Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string; see Matcher. . You need four backslashes: Two backslashes for declaring the String in Java (that will be one backslash in the actual string), and you need two backslashes in the regular expression as a backslash again is a special character for the regex engine: original. More importantly, groups are numbered according to their position in the regex, so using non-capturing groups whenever possible makes it a lot easier to keep track of the capturing-group numbers. )* would be gratuitously inefficient. In literal Java strings the backslash is an escape character. Remove everything between specific brackets. Matcher matcher = pattern. There is no need to enclose the whole consuming pattern with capturing parentheses, $0 backreferences stands for the whole match value. e. : foo,bar,foo,bar. On the other hand, the first String arguments of replaceFirst and replaceAll are regular expressions (regex). String test = "This is my test ( inside the brackets ) and finish here"; String regex = "\\(. Jan 10, 2017 · I am working with a regex and I want to use it on the replaceAll method of the String class in Java. You need more backslashes in the replacement text. You can use lookarounds in your regex: String repl = str. Approach 1 : First point is that I think a better approach to fixing this would be to simply have the application be aware of the set of databases and have it send a query The replaceAll() method searches a string for a value or a regular expression. NET, Java, Perl, PCRE, JavaScript, Python, Ruby Dec 10, 2013 · The $1 is a "backreference", referring to the first "capture group". matcher(stringInput)) Sep 26, 2016 · In this case b. This, however, means that the backslash is a special character, so if you actually want to use a backslash it needs to be escaped. *\. Both techniques offer unique advantages for string manipulation, and understanding them can significantly improve your regex proficiency. S. String regex = "\\b(\\w+)(\\s+\\1\\b Apr 16, 2015 · You are passing literal texts as startsWith and endsWith, thus you need to quote these values when building a dynamic regex pattern with RegExp#quote(java. 1 + q Compare the replacement text syntax and features supported by all the major regular expression flavors, including . util. Regular expressions are greedy by default, meaning that the first group captures as much as possible without violating the regex. matcher("2==2 Sep 12, 2008 · @Paramaleon If it did work by adding individual escapes, your initial example still wouldn't do what you wantedif it escaped characters individually, it would turn *. Dec 15, 2015 · I am running into problems replacing these string using numbered backreferences. When dealing with groups in a regular expression, backreferences provide a way to match previously captured text without re-writing the entire pattern again. Java version: 1. g. this May 12, 2014 · you can use Java - String replaceAll() Method. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand Feb 19, 2014 · (\d)\d\1 step by step:. I would want the regex to match other words like "teeth". [A-Za-z\\s\\. 正则表达式 [ ](\w+)[ ]\1 执行结果. And later in Matcher. Exception in thread "main" java. *)", "$1"); Jul 13, 2012 · Replace all doubled or tripled letters with single ones Hot Network Questions How to type this mathematical symbol, it is not mathbb{d} Aug 20, 2013 · I have the following string: String str = "Klaße, STRAßE, FUß"; Using of combined regex I want to replace German ß letter to ss or SS respectively. In your examples, the result of . NET, Java, Perl, PCRE, JavaScript, Python, Ruby Mar 30, 2010 · Well, if you want to use regex, then you can use replaceAll. Following code snippet replace all but one repeated characters System. quote(matchingStr), replacementStr) But as Jon says you can just use replace(). They're good, they're fast, and I only use them when I completely control the input being fed into the regex. * can match anything. Throws: PatternSyntaxException - If the syntax of the regular expression is incorrect. replaceFirst("(\\d{8})\\d{6}","$1000000");? Java treats $10 as "backreference 1, followed by a literal 0" if there are fewer than 10 capturing groups in your regex. How to replace all but 2 consecutive repeat Sep 12, 2023 · A backreference is a way to match the same text as previously matched by a capturing group. and Pattern. But beside that, what exactly is not working correctly with String newstring = timestamp. out. – Jun 7, 2014 · I have created simple demo to see how backreferences work. Dec 23, 2024 · The String replaceAll method in Java replaces substrings matching a specified regex pattern with a given string, but it throws exceptions for invalid or null regex inputs. to PC words and O. If you write . DOTALL modifier. so for example if the string given was abc123efg and they want me to replace every character except every instance of 123 then it would become +++123+++. 2. compile("([0-9]+)([,=]+)\\2\\1"); Matcher matcher = pattern. It's matching everything before a new line with 5 or more "=" ([^]+?)\n[=]{5,} Now I need to replace all double empty lines in the matching group. ) Jan 6, 2009 · He details the limitations in his great answer "Java Regex Helper" Java 7 regex named group support was presented back in September 2010 in Oracle's blog. quoteReplacement; References. Nov 26, 2012 · String replaced = myString. Description from codingbat. If you need only characters (not numbers) you can use the regular expression [^\\p{L}]+ as follow: replaceAll(String regex, String replacement) Replaces each substring of this string that matches the given regular expression with the given replacement. $12 is replaced with the 12th backreference if it exists, or with the 1st backreference followed by the literal “2” if there are less than 12 backreferences. This means that it takes the first group. The regular expression \\ matches a single backslash. CASE_INSENSITIVE). The two methods produced the same result in the above example since the characters in the regex pattern “hello” have no special meanings. Aug 6, 2018 · ReplaceAll is treating it's input as a regular expression. Now last problem you will face is second argument of replaceAll method. 1. Jan 19, 2022 · An invocation of this method of the form str. Finite Repetition Aug 12, 2013 · The regex in your code sample is syntactically invalid. You ask to replace all occurrences: the first occurrence does match the whole string, the regex engine therefore starts from the end of input for the next match; You could use the following regular expression to find parentheticals: \([^)]*\) the \(matches on a left parenthesis, the [^)]* matches any number of characters other than the right parenthesis, and the \) matches on a right parenthesis. My initial thought was to use backreferences, something like (foo|bar)(,\1)* But it turns out that this regex only matches foo,foo or bar,bar but not foo,bar or bar,foo (and so on). Syntax. Matcher; regular-expressions. r val REGEX_NEW_LINE = """\\\\\\n Sep 27, 2023 · I added the backreference \1 to test the 1st capturing group. The first \d matches one digit; And the parentheses mark this as a capturing group - this is the first one, so the digit is remembered as "group 1"; The second \d says there is another digit Jul 13, 2015 · You can use this replaceAll approach in Java if you do not want (we'll use the backreference $1 to this group to retrieve the text Java regular expression Feb 6, 2015 · This is similar to my previous efforts (wordEnds and repeatEnd): as a mental exercise, I want to solve this toy problem using regex only. match is an array if the pattern matches and the second element will contain the content of the first capture group, if that is what you want. I believe this can be done with regular expressions, but I don't know how to do it yet. Apr 19, 2012 · Assuming your string contains the entire word: String resultString = subjectString. The replaceAll() method does not work in Internet Explorer. To check if it is the same, we can capture the first character and use a backreference to match the same text, and a limiting quantifier {3,} to check for at least 3 occurrences of the same substring. separator"), " "); If I create lines in "string" by using "\n" I had to use "\n" in the regex. That Apr 25, 2017 · Due to my poor Java knowledge I am unable to manage 3 and 4. Regex replaceAll on a string. path is a Windows path containing \ it may well have invalid escape sequences. wav into the regex pattern \*\. Jan 23, 2016 · Backreference is a way to repeat a capturing group. a is the backreference, Java String. replaceAll. replaceAll("paloalto\\\\", "sanjose\\\\"); Jul 6, 2010 · To match an arbitrary literal string that may contain regex metacharacters, use Pattern. In contrast, replaceAll() uses regex to match patterns, making it a more advanced tool for string manipulation. The regex I posted should work either way, though ((?!\1). <b> Compare the replacement text syntax and features supported by all the major regular expression flavors, including . Pattern pattern = Pattern. appendReplacement method is adding to StringBuffer all data until current match, and lets you specify what should be put in place of matched by regex part When replacing characters using regular expressions, you're allowed to use backreferences, such as \1 to replace a using a grouping within the match. replaceAll("$1$2"). There are some valid escape expressions in regex, like \w for a word character or \s for any whitespace. IndexOutOfBoundsException: No group 1 Try using this command with this regular expression. Say you want to replace all vowels in a string with a parenthesis followed by the vowel followed by another parenthesis. This method replaces each substring of this string that matches the given regular expression with the given replacement. replaceAll( "(?x) # Multiline regex:\n" + "m # Match a literal m\n" + "( # Match and capture in backreference no. I think the matching regex for that is: [^A-Za-z][A-Za-z]\\. Edit May 11, 2021 · In Java Regex (?<tagName>MATCH_ME) defines a named group matching MATCH_ME; In Java Regex \\k<tagName> refers to the already named group (here MATCH_ME) In Java replacement ${tagName} refers to the named group captured from the Regex. replaceAll("\\\\+", "\\") and it will find match for regex you will see exception. Specifically you need to define your replacements in a Map , use a StringBuilder (before Java 9, less performant StringBuffer should have been used) and the appendReplacements() and appendTail Dec 8, 2009 · (You should not parse HTML with a regex) Two things: Java uses $1, $2 to refer to capture groups, not \1, \2. 1 + q Jan 19, 2022 · An invocation of this method of the form str. 4. The mere fact that you put the word boundary pattern inside a capturing group does not mean the word boundary semantics are passed with the backreference. ” with colons “:“. lang. For nested capturing groups, the order of the groups is the same as the order of the left parentheses. Capturing groups count from 1, so the first capturing group's result can be referenced with \1, the second with \2, and so on. Feb 1, 2016 · In Java, and it seems in a few other languages, backreferences in the pattern are preceded by a backslash (e. It is. replaceAll(regex, replacement) states: Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string; see Matcher. String input). They are created by placing the characters to be grouped inside a set of parentheses – †()†. info. Here's a snippet to illustrate: "left-right". replaceAll(". Problem is I can not replace all but two repeated consecutive characters. wav, and the replaceAll would turn it into \. C. The replaceAll() method returns a new string with all values replaced. ][^A-Za-z] How can I replace only the dot from that and not everything that matches? EDIT: Expected Jul 24, 2012 · Thanks for the reminder about the syntax for coding a backreference into a replacement string, which I wasn't finding elsewhere. The replaceAll() method does not change the original string. public String replaceAll(String regex, String replace_str) Parameters: regex: the regular expression to which this string is to be matched. Here is the syntax of this method: public String replaceAll(String regex, String replacement) Here is the detail of parameters: Mar 30, 2010 · Well, if you want to use regex, then you can use replaceAll. In my scenario the leading four digits will always be four digits followed by the letters cc so I figure this can be done with regex. replaceAll() support multiple backreferences at all? If so, what is the proper syntax to use it? A copy of the string in which matches of the regular expression are replaced with new substrings. public static String replaceGroup(String regex, String source, int May 11, 2021 · In Java Regex (?<tagName>MATCH_ME) defines a named group matching MATCH_ME; In Java Regex \\k<tagName> refers to the already named group (here MATCH_ME) In Java replacement ${tagName} refers to the named group captured from the Regex. Does Matcher. To perform this I have: String replaceUml = st Mar 1, 2012 · I am trying to write a regex which would match a (not necessarily repeating) sequence of text blocks, e. Replacement strings may contain a backreference in the form $n where n is the index of a group in the pattern. replaceAll("- ","")); or Apr 3, 2019 · You may use . replaceAll("([a-zA-Z])\\1{2,}", "$1")); Help is required to find out A. java regular expression with replaceAll on string. The first level of backslashing is consumed by the Clojure reader because it's a literal string. replaceAll() with back reference. Strings in Java are immutable, which makes this somewhat tricky if you are talking about an arbitrary number of things you need to find and replace. Syntax: public Matcher reset() Parameters: Sep 15, 2015 · The documentation of String. The documentation of Matcher. Also, your second reference is one of my go-to sites for reminders and cheat sheets. If the optional group is not null, it takes a numbered reference, and $1\s$3$4 returns 1 240x. *\\)"; ReplaceAll is a Java String class method: public String replaceAll(String regex, String replacement) Replaces each substring of this string that matches the given regular expression with the given replacement. The second level of backslashing is consumed by the regex. 0. wav, meaning it would match files whose name consists of a arbitrary number of periods followed by . replaceAll("cc","CC"); because it would replace all the occurrences of cc with capital versions so the word accelerator would become aCCelerator. Aug 11, 2012 · The problem here is not the regular expression, but the replacement: $ is used to refer to matching groups. how to replace all instances of "[" and "]" 0. *)", "$1"); Aug 28, 2014 · I want to replace all the P. replaceAll(regex, repl) is equal to Pattern. toString() from the model to the view, and I want do display it using html tags in a pretty way. (Note that the first parameter of this method is a regular expression. matcher(str). I want to replace all the matches with part of the matching regex (group). Oct 6, 2014 · You're on right track and your regex looks good. 注意，\w+是出现在括号里，所以它是一个子表达式，该子表达式对模式进行分组，将其标识出来以备候用。 Apr 17, 2014 · @Casimiret, nice trick. val REGEX_OPEN_CURLY_BRACE = """\\{""". Replace all characters in a string with + symbol except instances of the given string in the method. If you're including this in a java string, you must escape the \ characters like the following: String regex May 31, 2012 · The replace method will replace all occurrences of a char or CharSequence. 1\n" + " [a-d] # One letter from the range a through d\n" + ") # End of capturing group\n" + "$ # Assert position at the end of the string", \ "$1q"); // replace with the contents of group no. Group in regular expression means treating multiple characters as a single unit. Compare the replacement text syntax and features supported by all the major regular expression flavors, including . In general, I want to replace all the dots that are between single letters OR single letter and a space or dot. We'll compare the back reference and lookaround methods for this operation and evaluate their performance. I obviously can't do text. To accomplish this task we will definitely need the replace() method, since we need to perform replacements. Mar 4, 2021 · Regex to replace all non numbers but allow a '+' prefix (since $0 is a backreference to the whole match Java: Replace all non-digit symbols except symbol Nov 21, 2017 · You could use Matcher#start(group) and Matcher#end(group) to build a generic replacement method:. G to OMG. When Java compiler sees two slashes, it replaces them with a single slash. Jul 19, 2012 · I am trying to create a regex in Java to match the pattern of a particular word to find other words with the same pattern. But I haven't been able to make it work with two or more backreferences, e. replaceAll("\\\\"", ""); Nov 26, 2021 · Hi, I am Amr Hesham a Software Engineer, I am interested in Android Development and Compiler Design, In this article, I will talk about very good and useful feature which is Regex Named group and Backreferences with examples, Regex Named group and Backreferences introduced first time in Python re module then Microsoft developers supported it in May 4, 2015 · The programming language is Java. Feb 10, 2014 · I used Pattern as parameter in replaceAll method, I want to remove all characters inside open and close bracket (include bracket characters) however only inside chars was remove, the bracket is still there. $0 (dollar zero) inserts the entire regex match. Sample Text: Hello World. g Feb 7, 2017 · The regular expression [^\p{L}\p{Nd}]+ match all characters that are no a unicode letter or a decimal number. r val REGEX_INLINE_DOUBLE_QUOTES = """\\\\\\"""". I have no possibility to change the Java code, the only thing I can change is the matching group from the result and the regex itself. replaceAll(System. compile(regex). getProperty("line. The replaceAll() method replaces the first match of a regular expression in a string with a new substring. toString(). When you use regular expressions to replace part of a string, you can use $1 in the replacement string to refer to the first capture group. Jan 21, 2017 · As such, you want a regular-expression to remove the offending part of the query - which is in the order-by clause for the two target databases where it does not work. replaceAll(regex, repl) yields exactly the same result as the expression. replaceAll("(?<!^)\\*+(?!$)", ""); RegEx Demo. replaceAll, and explicitly creating a Matcher and Pattern should be the same. appendReplacement(StringBuffer, replacement) and matcher. But your regular expression is only $1. Mar 15, 2010 · To clarify: $1 and $2 are user-chosen parameter names here (chosen to mimic the backreference symbols); the - varying! - number of these parameters corresponds to the number of capture groups in the regex. See this Java demo. *)-(. quote; To replace with an arbitrary literal string that may contain replacement metacharacters, use Matcher. The replaceAll() method was introduced in JavaScript 2021. replaceAll() with a single backreference works great (i. This solution uses a reluctant quantifier and a backreference: This solution uses a reluctant quantifier and a backreference: String cleanxml = rawxml. Nov 11, 2012 · Use replaceAll(String replacement) API method, with a given String parameter to replace all subsequences of the sequence that matches the pattern with the given String, as described in the code snippet below. Nested capturing groups. replaceAll("(?i)"+label, "WINDOWS"); Note: If the label could contain characters with special regex significance, eg if label was ". 4 Nov 6, 2024 · Regular Expressions, Literal Strings and Backslashes. Using the wrong function can lead to subtle bugs. sb. replaceAll("\\s+\\$\\$", "\\$\\$"); Sep 30, 2012 · Java replaceAll Regex for special character [2. Now next steps are: Compile the string regex to make a Pattern object; Create Matcher instance by passing in your string input to compiled Pattern object (i. Pattern. com: Given a string and a non- Sep 7, 2021 · The reset() method of Matcher Class is used to reset this matcher, in order to understand it better it is recommended to have prior knowledge of Pattern and Matcher class in java regex sub-package. M. So, when I try to replace my text using backreference pointing to the eleventh group, I am getting the first group with a "1" attached to it, instead of the group eleven. replaceAll(repl) Using backreferences to preserve case: Jun 5, 2013 · Because the replacement string has the capacity to backreference the matches from the matcher pattern (using $), and as a result you also need to treat ` specially because it is used to escape $` characters (to make them literals instead of backreferences). Regex의 Metacharacters, Quantifiers, Grouping에 대해서 정리하였고 다양한 예제로 설명합니다. replace_str: the string which would replace found expression. Pattern; java. You need to escape it with a second \. Jun 28, 2012 · Just add the "case insensitive" switch to the regex: html. Nov 6, 2013 · Backreferences in Java Regular Expressions is another important feature provided by Java. Oct 19, 2020 · Java RegEx: Part 8 — Backreferences. Here we will be illustrating it with help of Java programs. Oct 23, 2011 · Matcher. As such, the values that are passed to your header() method are the $0, $1 and $2 literals and not the captured values.