RegEx

Steven RogerMay 7, 2024

0 31 3 minutes read

The name of Regex is derived from Regular Expression. The regex or regular expression will be a series of various characters which explains a specific pattern of search.It is shown as Rational expression mainly for searching and managing text strings. It is easily used to search a pattern which we can replace with the matching pattern.

This tool is made use in all the programming languages as well as scripting language like PHP, C, C++, Java, Perl and javaScript, ruby and python. It is mainly used as word processors like word helps the clients users who searches the text in a document.

Table of Contents

Regular Expression characters:

The following are the different type of characters of a regular expressions

Metacharacters
Quantifiers
groups and ranges
Escape characters

The Metacharacters are:

^ : This character is used to match a expression that is right at the starting of any string. For example ^a will be an expression which matches the string that starts with ‘a’ such as “aab”
$: This sign is employed to match an expression to its left at the end of the string. For example r$ is an expression to match to a string which will end with an symbol r like “aaabr”, ”ar”, ”r”.
. : This is the character that is employed to match any single character string without the terminator line as /n. For example b.x will be an expression that will match strings like “bax”, ”bar”.
| : It is a character matched for a particular group of characters on either side. If the character is on the left side that is matched then the right side character will be ignored. For example A|B is an expression that will give many strings but all the strings contain either a or b.
\ : It is used specifically to escape the special character after this sign in a string.

Quantifiers

The quantifiers are used for all regular expressions for telling the number of occurrences of a character.

The quantifiers

+ -: This character will be specifying an expression to the left for more times. For example s+ is an expression which gives “s” ,”s”.
? -: This character mainly specifies an expression to its left for 0 or 1. For example aS? is called an expression which gives either “a”.
* -: This character will make out an expression to the left for 0 or more times. For example Br* will be an expression which gives “B”,”Brrr” etc.
{x} -: It mentions an expression to its left for only x times. For example Mab{5} will be an expression which provides the following string which contains 5 b’s Mabbbbb.
{x,} -: It says an expression to the left side for x or more times. For example Xb{3,} is an expression which gives various strings containing at least 3 b’s. Such strings as “Xbbb” and so on.
{x,y} -: It will say an expression to its left at least x times but less than y times. Consider an example, Pr{3,6} a is an expression which provides two strings Both strings will as follows “Prrrr”.

Escape characters:

\s –It is employed to match a one white space character.
\S –It matches one white space character.
\0 –It matches Null character.
\a –It matches bell or alarm
\d –It will be used to match one decimal digit ,which ,means 0 to 9.
\D –It matches any decimal digit.
\w –It matches the alphanumeric characters.
\W –It matches one non-word character.

Regular expressions in Java language:

In Java language, Regex will be an expression for application programming interface which is used for manipulating, searching and editing a string. We have subsequent classes which comes under the java.util.regex package

regex.pattern- util.regex.pattern this class will be compiled version of Regex and called by the compile() method. The compile() method that accepts the regex as a first argument. This class does not have public constructor.

Example

import java.util.regex.*;

class reg{

public static void main(String args[]){

System.out.println(Pattern.matches(“.r.”, “app”)); // This line displays Boolean value True because the second character is r in both string and regular exp.

System.out.println(Pattern.matches(“.bm”, “abc”)); //here the Boolean value is false as the third character will be different in regular expression and strings

System.out.println(Pattern.matches(“..m”, “mnm”)); //This line displays Boolean value True as the third character is m in string and regular exp.

System.out.println(Pattern.matches(“a..s”, “asns”)); //This line displays Boolean value True because the first and last character is same in both string and regular exp.

System.out.println(Pattern.matches(“.s.”, “mds”)); //Line displays Boolean value False because the second character is different in both string and regular exp.

}