{"id":27161,"date":"2020-12-17T19:52:04","date_gmt":"2020-12-18T03:52:04","guid":{"rendered":"https:\/\/careerkarma.com\/blog\/?p=27161"},"modified":"2022-07-20T08:57:52","modified_gmt":"2022-07-20T15:57:52","slug":"java-regex","status":"publish","type":"post","link":"https:\/\/careerkarma.com\/blog\/java-regex\/","title":{"rendered":"Java Regex: An Introduction"},"content":{"rendered":"\n

There may come a time in your application development journey where you need to find patterns in strings. Some methods can take time to execute, while others save you time.
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One time-saving technique called regex \u2014 regular expressions \u2014 can be used to find or validate string patterns. This article takes a look at what regular expressions are, how they might be used in Java, and resources to help you with writing Java-flavored regex. <\/p>\n\n\n\n

What are Regular Expressions?<\/h2>\n\n\n\n

At a very high level, regular expressions are just pattern matchers \u2014 characters that match a pattern. The actual metacharacters used to build a regular expression are fairly language agnostic for those languages that support using it. This means that the pattern in Java is most likely the same in JavaScript or Ruby.
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The methods in Java are fairly similar to those found in other languages as well. It\u2019s mainly the syntax that\u2019s different.<\/p>\n\n\n\n

What are Regular Expressions Used for?<\/h3>\n\n\n\n

A Regular Expression is useful when you need to search and replace a pattern in a string, and when you need to validate a form. Depending on the circumstances, you can test your regex pattern in a number of different ways.<\/p>\n\n\n\n

Search and Replace<\/h4>\n\n\n\n

The first use case for using regular expressions would be if you want to search for a particular pattern and then replace it with something else. 
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Let\u2019s say we have a customer database that has some email address for each customer:<\/p>\n\n\n\n

Sample Customer Database<\/h4>\n\n\n\n
id<\/strong><\/td>first_name<\/strong><\/td>last_name<\/strong><\/td>email<\/strong><\/td><\/tr>
1<\/td>Doris<\/td>Symcox<\/td>dsymcox0@elpais<\/td><\/tr>
2<\/td>Linnell<\/td>Friar<\/td>lfriar1@pinterest.com<\/td><\/tr>
3<\/td>Paulie<\/td>Mathes<\/td>pmathes2hud.gov<\/td><\/tr>
4<\/td>Davina<\/td>Boam<\/td>dboam3@people.com.cn<\/td><\/tr>
5<\/td>Hulda<\/td>Coneybeare<\/td>hconeybeare4@taobao.com<\/td><\/tr>
6<\/td>Jaymee<\/td>Barnes<\/td>jbarnes5@vistaprint.com<\/td><\/tr>
7<\/td>Wendy<\/td>Fley<\/td>wfley6@unesco..com<\/td><\/tr>
8<\/td>Torr<\/td>Rustich<\/td>trustich7@java.com<\/td><\/tr>
9<\/td>Norene<\/td>Redwing<\/td>nredwing8@umn.edu<\/td><\/tr>
10<\/td>Philbert<\/td>Merveille<\/td>pmerveille9@jimdo.com<\/td><\/tr><\/tbody><\/table>\n\n\n\n

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If you take a quick look at the database, you can see there are some typos in the email field. Now imagine having 1,000 of those fields! It would be too timely to check over each record by hand. 
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If we are unsure as to whether or not all of the addresses are valid email addresses, we can use regex methods to make sure it has the correct format. If it does not, we can replace it with something else \u2014 either a null value or something of your choosing to indicate that the email is incorrect.<\/p>\n\n\n\n

Validation<\/h4>\n\n\n\n

The other way to use regular expressions is to validate something. When we validate, we want to make sure it follows the correct format. This is an optimal time to make sure a user is giving you the proper format for their input fields.
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Take, for instance, when a user inputs a phone number into a form. You can use regex to write a function that makes certain that the input from the user is in the format we want. When working with databases, it\u2019s important to have the same format for all the fields. It makes working with the data much easier.
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For validators, you can either raise an error to the user that the input needs to be entered in a certain format or you can write a function that will take the user\u2019s input and save it in the format you\u2019d like. This is fairly easy to do if you use regular expressions.<\/p>\n\n\n\n

Types of RegEx Pattern Matchers<\/h3>\n\n\n\n

There are several different types of matchers that will assist you with writing your regular expressions, which include: literal characters, metacharacters, and quantifiers.<\/p>\n\n\n\n

Literal Characters<\/h4>\n\n\n\n

The most simple example of a regex is a literal string. The string \u201chello\u201d can be an example of a regex, as can this whole paragraph.
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Examples: 
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\/hello\/<\/code> \u21d2 collection of five distinct characters.
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When a regex pattern is applied here, it looks for each of these characters in succession. \u201chello\u201d, \u201chelloing\u201d, or \u201chelloed\u201d passes, but \u201cHello\u201d, \u201chelo\u201d, or \u201cHeLlo\u201d would not.  
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\/The most simple example of a regex is a literal string. The string \u2018hello\u2019 can be an example of a regex, as can this whole paragraph\\.\/<\/code> \u21d2 collection of characters that make up a paragraph can also be a regex. 
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The same applies to this second example. The pattern looks for exactly that arrangement of characters in that exact same way when it\u2019s tested against a string.
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When a test is failed, the terminal will throw an error when it\u2019s executed. 
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Take another look at the second example. Do you notice the forward slash (\\) next to the period (.)? This is what it means to escape<\/em> a character. For the regex engine to overlook that and actually see it as a period, we have to escape it with a forward slash. The period, or dot, we will learn is a defined pattern matcher in regex.
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Here are some other characters that need to be escaped if you want the literal character instead of the translated meaning that the regex engine compiles it to. 
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