Often unknown, or heralded as confusing, regular expressions (regex) have defined the standard for powerful text manipulation and search. Without them, many of the applications we know today would not function. This two-part series explores the basics of regular expressions in Java, and provides tutorial examples in the hopes of spreading love for our pattern-matching friends. (Read.) Part 1: What are Regular Expressions? Regular expressions are a language of string patterns built in to most modern programming languages, including; they can be used for: searching, extracting, and modifying text. This chapter will cover basic syntax and use. Maxtor Repair Tools Download more.

Java.util.regex.Matcher.group() Method Example - Learning Java Regex in simple and easy steps. A beginner's tutorial containing complete knowledge of Overview, Capturing Groups, MatchResult, Pattern, Matcher, PatternSyntaxException, Examples, Characters,Character Classes, Predefined Character Classes, Boundry. Each pair of parentheses in a regular expression defines a separate capturing group in addition to the group that the whole expression defines. Package org.kodejava.example.util.regex; import java.util.regex.Matcher; import java.util.regex.Pattern; public class CapturingGroupDemo { public static void.

This article is part one in the series: “[[Regular Expressions]].” Read for more information on lookaheads, lookbehinds, and configuring the matching engine. To get a more visual look into how regular expressions work, try our. You can also watch a video to 1. Syntax Regular expressions, by definition, are string patterns that describe text. These descriptions can then be used in nearly infinite ways.

The basic language constructs include character classes, quantifiers, and meta-characters. Character Classes Character classes are used to define the content of the pattern.

What should the pattern look for? Dot, any character (may or may not match line terminators, read on ) d A digit: [ 0 - 9 ] D A non -digit: [ ^ 0 - 9 ] s A whitespace character: [ t n x0B f r ] S A non -whitespace character: [ ^ s ] w A word character: [a -zA -Z_0 - 9 ] W A non -word character: [ ^ w ]. Dot, any character (may or may not match line terminators, read on) d A digit: [0-9] D A non-digit: [^0-9] s A whitespace character: [ t n x0B f r] S A non-whitespace character: [^ s] w A word character: [a-zA-Z_0-9] W A non-word character: [^ w] However; notice that in Java, you will need to “double escape” these backslashes. Escape the next meta -character (it becomes a normal /literal character ) ^ Match the beginning of the line. Match any character (except newline ) $ Match the end of the line ( or before newline at the end ) Alternation (‘ or’ statement ) ( ) Grouping [ ] Custom character class Escape the next meta-character (it becomes a normal/literal character) ^ Match the beginning of the line.

Match any character (except newline) $ Match the end of the line (or before newline at the end) Alternation (‘or’ statement) () Grouping [] Custom character class Visual Regex Tester To get a more visual look into how regular expressions work, try our. Examples 2.1. Basic Expressions Every string is a regular expression.

For example, the string, “I lost my wallet”, is a regular expression that will match the text, “I lost my wallet”, and will ignore everything else. What if we want to be able to find more things that we lost? We can replace wallet with a character class expression that will match any word.

'I lost my w+' 'I lost my w+' As you can see, this pattern uses both a character class and a quantifier. “ w” says match a word character, and “+” says match one or more. So when combined, the pattern says “match one or more word characters.” Now the pattern will match any word in place of “wallet”. “I lost my sablefish”, “I lost my parrot”, but it will not match “I lost my: trooper”, because as soon as the expression finds the ':' character, which is not a word character, it will stop matching.

If we want the expression to be able to handle this situation, then we need to make a small change. ( ) group everything within the parenthesis as group 1 mouse match the text ‘mouse’ alternation: match any one of the sections of this group cat match the text ‘cat’ //.and so on () group everything within the parenthesis as group 1 mouse match the text ‘mouse’ alternation: match any one of the sections of this group cat match the text ‘cat’ //.and so on 2.5. Modifying/Substitution Values in text can be replaced with new values, for example, you could replace all instances of the word ‘clientId=’, followed by a number, with a mask to hide the original text. (See below) For sanitizing log files, URI strings and parameters, and form data, this can be a useful method of filtering sensitive information. A simple, reusable utility class can be used to encapsulate this into a more streamlined method.

(clientId = ) group everything within the parenthesis as group 1 clientId =match the text ‘clientId =’ ( d + ) group everything within the parenthesis as group 2 d + match one or more digits (clientId=) group everything within the parenthesis as group 1 clientId=match the text ‘clientId=’ ( d+) group everything within the parenthesis as group 2 d+ match one or more digits Notice how groups begin numbering at 1, and increment by one for each new group. However; groups may contain groups, in which case the outer group begins at one, group two will be the next inner group.

When referencing group 0, you will be given the entire chunk of text that matched the regex. ( ( ) ( ( ) ( ) ) ) ( ) //and so on 1 2 3 4 5 6 //0 = everything the pattern matched ( ( ) ( ( ) ( ))) ( )//and so on 1 2 3 4 5 6 //0 = everything the pattern matched 3. Conclusion & Next Steps Wrapping up, regular expressions are not difficult to master – in fact, they are quite easy.

My strategy, whenever building a new regular expression, is to start with the simplest, most general match possible. The Magician Raymond E Feist Pdf here. From there, I continuously add more and more complexity until I have matched, substituted, or inserted exactly what I need.

Don’t be afraid to “express” yourself! When you’ve got the hang of these techniques, or need something a little fancier, read for more information on lookaheads, lookbehinds, and configuring the matching engine. About the author: is the Chief Editor of, and has worked extensively on open-source projects; most notably as creator & project lead of, author of, and Project Lead of. This content represents his personal opinions, not those of his employer. He is a founder of, the author of and, the leading URL-rewriting extensions for Servlet, Java EE, and Java web frameworks; he is also the author of, social-style date and timestamp formatting for Java. When he is not swimming, running, or playing competitive Magic: The Gathering, Lincoln is focused on promoting open-source software and making technology more accessible for everyone. Posted in 67 Comments •.

While it would seem tempting to implement this using a single regular expression (which is certainly possible), I would recommend splitting this up into 4 individual checks, with unit tests for each check. In this situation, clarity should be preferred over brevity, and the regular expression you want to construct will be a bit opaque if you attempt a one-liner. Performance is not really an issue for something like this (unless you have some strange requirements or expectations: This is really pretty easy, so I’ll give you this code under one condition – you have to post a link on a blog back to this article!

I would suggest that references be provided to native regular expression man pages. I would think that the way to really understand regex’s would be to understand native regex’s as they would be used in sed or egrep or other standard Unix utilities, and then understand what the java library limitations are if any. I note that just about the first thing you do in part one, is talk about the escaping for backslash in pattern definition strings. I don’t know if there would be a way to do this, given that it appears that operator overloads appear to be not possible in Java, but I think a useful capability would be a java library/module (like prettytime) that linguistically overloads say the tick (single quote) or slash character so that regular expressions could be defined, and easier to read in java and consistent with other non-java examples. (cf: the perl slash (/pattern/) regex delimiter) which is the same as that used by standard Unix utilies like sed. I would think this would make generic regex man pages much more useful to the java user, increase the readability of regex’s in java, and as a result, maybe increase the general understanding and suffistication of regex usage by java programers. Unfortunately, operator overloads are not possible in Java, and there is no way to override the default behavior of the escape character in string literals, but I agree, it would be nice 🙂 Since this is a Java-targeted article, I did link to – the official regex docs.

I think linking to man pages here would end up being confusing because the syntax is subtly different regarding escaping and configuration. In general, Java regexps are a full implementation of Unix regex, but not as comprehensive as say, PCRE in a few ways. This can, however, be made up for via programmatic usage of the Pattern and Matcher classes. Hi Lincoln, Great Information. I want to do something reverse of this. I have to make my password rule to be configure by user through property file with RegEx value, then i have to validate the password value against configured RegEx.

I am success ed to validate it but now i have to also show, what is correct password format to the user so that he can correct it accordingly, how can i parse regex & find that it looks for n number of special char, n number of upper case alphabet, n number of numeric character? Thanks, -Sachin. Hello Thanks for your good information. I think a mistake is in one of the above examples!

For pattern '^I lost my:? (wallet car cell phone marbles)$' you mentioned some matches strings as: 'I lost my wallet' 'I lost my wallets' 'I lost my: wallet' 'I lost my: wallets' 'I lost my car' 'I lost my car' 'I lost my: car' 'I lost my: car' 'I lost my cell phone' 'I lost my cell phone' 'I lost my: cell phone' 'I lost my: cell phone' 'I lost my marbles' 'I lost my marbles' 'I lost my: marbles' 'I lost my: marbles' But second and fourth expression are not matching because they have an extra character ‘s’ at the end of the expression which is not allowed in the pattern 🙂.

@KitsuneYMG: Java's lookbehind is fixed-width. You can use (? Yes, as long as the pattern is /some text/ it's okay, but if you search for /^[0-9]+ s( w+)/ or the like, this will obviously break.

Another approach (that would not break likewise) would be to append a greedy.* in the beginning of the pattern, and truncate searched line at each found match, but finding the actual offset would become more problematic (the.* will always match at 0), so you'd have to substract length of your real match from matcher.end() Not very efficient again. – Mar 2 '10 at 11:05.