We are going to learn about\u00a0 Regular expressions in Java under the following topics:<\/p>\n
<\/p>\n
A regular expression is a special sequence of characters. Its also known as regex<\/strong>. You can use them to represent patterns in a text. Moreover, you can also used them to perform other operations. These includes: searching for particular pattern in a text or replacing occurrence of certain string.<\/p>\n For example, you may want to search for phone numbers inside several pages of text. Regular expression comes to the rescue in cases like this.<\/p>\n In Java, we have three classes used to work with regular expressions. This classes are available in java.util.regex package. They are Pattern, Matcher and PatternSyntaxException classes. Let’s now discuss each of them.<\/p>\n <\/p>\n There are three of such classes. We would consider them in this section. They are:<\/p>\n <\/p>\n Pattern Class<\/strong><\/p>\n So you can create a Pattern object from a pattern class. Hence,\u00a0 pattern object represents a particular regular expression. However, the Pattern class has not public constructors. As such, you cannot just create a Pattern object using new keyword.<\/p>\n How then can you create a pattern object?<\/p>\n You can do this by calling its static compile method(). This method accepts a regular expression as parameter and returns a Pattern object.<\/p>\n <\/p>\n Matcher Class<\/strong><\/p>\n Next class in the package is the Matcher class. The Matcher class helps you to match a string against a pattern. Just like the Pattern class, there are no public constructor in the Matcher class. However, you can create a Matcher object by calling the match() method of the Pattern object. Then you pass it the string\u00a0 you want to match.<\/p>\n <\/p>\n PatternSyntaxException<\/strong><\/p>\n A PatternSyntaxException object is an exception that occurs if there is error in the regular expression<\/p>\n <\/p>\n <\/p>\n So we are going to separate the syntax into categories. This is because there are quite a number of different expressions to use.<\/p>\n (a) Matching Characters<\/strong><\/p>\n <\/p>\n (b) Matching Boundaries<\/strong><\/p>\n So we provide a list of regular expressions that matches any boundaries.<\/p>\n <\/p>\n (c) Groups<\/strong><\/p>\n To work with groups you use braces. So the list is provided below.<\/p>\n <\/p>\n (d) Quantifiers<\/strong><\/p>\n Now we consider quantifiers. You use them to match more than one character at a time. Find the list below.<\/p>\n <\/p>\n Furthermore note that metacharacters must be escaped. Metacharacters include the following:<\/p>\n . [ { ( ) \\ ^ $ | ? * +<\/p>\n You escape a metacharacter by preceding it with a backslash (\\)<\/p>\n <\/p>\n <\/p>\n You can use the methods provided by the Matcher class to perform certain operations. So there are three categories of method we would discuss. They are:<\/p>\n <\/p>\n The index methods<\/strong><\/p>\n You use the index methods to get where a match was found. It provides the index position of the match. Returns an integer<\/p>\n This method returns the start index of the previous match.<\/td>\n<\/tr>\n Gives the start index of the subsequence captured by the given group during the previous operation.<\/td>\n<\/tr>\n Gives the offset after the last character matched.<\/td>\n<\/tr>\n Gives the offset after the last character of the subsequence captured by the given group during the previous operation.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n Verifier Methods<\/strong><\/p>\n So the verifier methods are used to check an input string to verify if a patter is found. Hence, it returns true if the pattern is found. Otherwise, it returns false. Find the list below.<\/p>\n Tries to match the input sequence, starting at the beginning of the region, against the pattern.<\/td>\n<\/tr>\n Tries to find the next subsequence of the input sequence that matches the pattern.\u00a0Returns true if it finds a match. Otherwise return false.<\/td>\n<\/tr>\n Resets this matcher and then tries to find the next subsequence of the input sequence that matches the pattern, starting at the given index. Returns true if there is a match. Otherwise return false.<\/td>\n<\/tr>\n Tries to match the entire region against the pattern. Returns true if\u00a0 it finds a match. Else it returns false.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n Replacement Methods<\/strong><\/p>\n This methods are used to replace matching text in the input string. Find the list below<\/p>\n Implements a non-terminal append-and-replace step. Returns a Matcher<\/td>\n<\/tr>\n Implements a terminal append-and-replace step. Returns a StringBuffer<\/td>\n<\/tr>\n Replaces every subsequence of the input sequence that matches the pattern with the given replacement string. Then returns the new string<\/td>\n<\/tr>\n Replaces the first subsequence of the input sequence that matches the pattern with the given replacement string. The returns the new string<\/td>\n<\/tr>\n Returns a replacement String for the specified String. This method produces a String that will work as a literal replacement\u00a0s<\/b>in the appendReplacement method of the Matcher class.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n <\/p>\n Let’s now apply what we’ve learnt so far.<\/p>\n We would\u00a0 write a regular expression that finds all 3 digit numbers in an input string<\/p>\n <\/p>\n2. Useful Classes in Regular Expressions<\/strong><\/h4>\n
\n
3. Syntax of Regular Expressions<\/strong><\/h4>\n
\n\n
\n Expression<\/th>\n Matches<\/th>\n<\/tr>\n \n .<\/td>\n \u00a0Any character except newline.<\/td>\n<\/tr>\n \n \\w<\/td>\n Word character (a-z, A-Z, 0-9, _)<\/td>\n<\/tr>\n \n \\W<\/td>\n Not a word character<\/td>\n<\/tr>\n \n \\s<\/td>\n Whitespaced character.\u00a0 Same as to [\\t\\n\\r\\f].<\/td>\n<\/tr>\n \n \\S<\/td>\n Non-whitespace.<\/td>\n<\/tr>\n \n \\d<\/td>\n Any digit. Same as to [0-9].<\/td>\n<\/tr>\n \n \\D<\/td>\n Nondigits.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \n\n
\n Subexpression<\/th>\n Matches<\/th>\n<\/tr>\n \n ^<\/td>\n Beginning of a string<\/td>\n<\/tr>\n \n $<\/td>\n End of a string<\/td>\n<\/tr>\n \n \\b<\/td>\n Word boundary<\/td>\n<\/tr>\n \n \\B<\/td>\n Non word boundaries.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \n\n
\n Expression<\/th>\n Matches<\/th>\n<\/tr>\n \n […]<\/td>\n Any single character in brackets.<\/td>\n<\/tr>\n \n [^…]<\/td>\n Any single character not in brackets.<\/td>\n<\/tr>\n \n a| b<\/td>\n Matches either a or b.<\/td>\n<\/tr>\n \n (re)<\/td>\n Groups regular expressions enclosed in brackets.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \n\n
\n Expression<\/th>\n Matches<\/th>\n<\/tr>\n \n *<\/td>\n 0 or more occurrences of the preceding expression.<\/td>\n<\/tr>\n \n +<\/td>\n 1 or more of the previous thing.<\/td>\n<\/tr>\n \n ?<\/td>\n 0 or 1 occurrence of the preceding expression.<\/td>\n<\/tr>\n \n { n}<\/td>\n Exactly n number of occurrences of the preceding expression.<\/td>\n<\/tr>\n \n { n,}<\/td>\n n or more occurrences of the preceding expression.<\/td>\n<\/tr>\n \n { n, m}<\/td>\n Range of numbers. Minimum of n and maximum of m<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n 4. Matcher Class Methods<\/strong><\/h4>\n
\n
\n\n
\n SN<\/th>\n Method name description<\/th>\n<\/tr>\n \n 1<\/td>\n start()<\/b><\/p>\n \n 2<\/td>\n start(int group)<\/b><\/p>\n \n 3<\/td>\n end()<\/b><\/p>\n \n 4<\/td>\n end(int group)<\/b><\/p>\n \n\n
\n SN<\/th>\n Method name and description<\/th>\n<\/tr>\n \n 1<\/td>\n lookingAt()<\/b><\/p>\n \n 2<\/td>\n find()<\/b><\/p>\n \n 3<\/td>\n find(int start)<\/b><\/p>\n \n 4<\/td>\n matches()<\/b><\/p>\n \n\n
\n SN<\/th>\n Method & brief description<\/th>\n<\/tr>\n \n 1<\/td>\n Matcher appendReplacement(StringBuffer sb, String replacement)<\/b><\/p>\n \n 2<\/td>\n StringBuffer appendTail(StringBuffer sb)<\/b><\/p>\n \n 3<\/td>\n String replaceAll(String replacement)<\/b><\/p>\n \n 4<\/td>\n String replaceFirst(String replacement)<\/b><\/p>\n \n 5<\/td>\n static String quoteReplacement(String s)<\/b><\/p>\n 5. Demo of Regular Expressions<\/strong><\/h4>\n
import<\/span> java.util.regex.Matcher<\/span>;<\/span>\r\nimport<\/span> java.util.regex.Pattern<\/span>;<\/span>\r\n\r\npublic<\/span> class<\/span> RegexDeom<\/span> {<\/span>\r\n\r\n\tpublic<\/span> static<\/span> void<\/span> main<\/span>(<\/span>String[]<\/span> args)<\/span> {<\/span>\r\n\t\tPattern pt =<\/span> Pattern.<\/span>compile<\/span>(<\/span>\"\\\\d{3}\"<\/span>);<\/span>\r\n\t\t\r\n\t\tString text =<\/span> \"23 becomes 230 if you add 0 \"<\/span> +<\/span>\r\n\t\t\"but its not same as 544 be it becomed 3\"<\/span>;<\/span>\r\n\t\t\r\n\t\tMatcher mt =<\/span> pt.<\/span>matcher<\/span>(<\/span>text);<\/span>\r\n\t\t\r\n\t\tint<\/span> count =<\/span> 0<\/span>;<\/span>\t\t\r\n\t\twhile<\/span>(<\/span>mt.<\/span>find<\/span>())<\/span> {<\/span>\r\n\t\t\tcount =<\/span> count+<\/span> 1<\/span>;<\/span>\r\n\t\t}<\/span>\t\t\r\n\t\tSystem.<\/span>out<\/span>.<\/span>println<\/span>(<\/span>count);<\/span>\t\t\r\n\t}<\/span>\r\n}<\/span>\r\n<\/pre>\n6. Watch the Video<\/strong><\/a><\/h4>\n