What is Regex? A Guide to Regular Expressions

What if you could have a unifying language that helps ease the cumbersome task of searching, matching and manipulating text patterns? The answer to that question lies in Regex, which is short for Regular Expressions. Think of it as a language within a language that helps developers, and Data Analysts sort through data or extract specific details from a complex document with ease.  

If you are curious to dig deeper, wait no more! This blog sheds light on What is Regex and what makes it such a pattern-matching powerhouse. So read on and revolutionise the way you handle texts! 

Table of Contents

1) What is a Regex (Regular Expression)? 

2) When to Use Regex 

3) Special Characters in Regex  

4) Regular Expression Flags 

5) How to Write a Regular Expression Pattern 

6)  Examples of Regular Expressions 

7) Regex Implementation in Java  

8) Popular Use Cases of Regex  

9) Conclusion 

What is a Regex (Regular Expression)? 

A Regex is a sequence of characters used to match a specific pattern in text. It may also be combinations of special character operators (symbols controlling the search) that we can use for advanced find and replace searches. The Regex is applied to the text from left to right. Once a source character has been utilised in a match, it can't be reused. For example, the regex ‘ded’ will match ‘dedededed’ only two times (ded_ded__).
 

When to Use Regex 

Regex is a pattern-matching process that must be used when the types of strings to be matched only conform to a particular pattern or are variable. When you have to conduct a simple string search, you can employ the built-in methods of the String class. Typical uses of Regex are:  

1) Input Validations 

2) String Parsing 

3) Syntax Highlighting 

4) Data Scraping 

5) String Manipulation 

6) Data Mapping 

Special Characters in Regex  

Regex patterns are made of a combination of characters and special symbols. Some fundamental Regex special characters and their meanings are summarised in the table below: 

Regular Expression Flags 

Regular expression flags refer to modifiers that change how the Regex engine interprets a pattern. Some common flags include: 

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How to Write a Regular Expression Pattern? 

To write a Regex pattern, you must define a specific sequence of characters that describes the text you seek to match. The following tools will help you write a Regex pattern with ease:
 

1) Assertions 

Assertions are special characters in Regex that remove partial matching or ambiguity from an expression. Without assertions, you could match a portion of the user's input to a document or field, yielding unwanted results. Assertions come as either anchors or lookarounds. The two most essential anchors are the caret and dollar sign: 

2) Quantifiers 

Quantifiers offer flexibility in matching as they define the number of times a pattern, character, or group appears in a Regex match. 

3) Metacharacters 

Metacharacters, or shorthand, are additional special characters that replace longer Basic Regular Expressions (BRE) expressions. The table below lists some important metacharacters: 

4) Escaping 

Escaping involves using a backslash () to give special characters a literal meaning instead of their predefined Regex functions. This is important when you match characters with special meanings in Regex. For example, if you want to match a literal dot (.) in a string, you can use . in your Regex pattern. 

5) Parentheses 

In Regex, parentheses () are used for grouping and capturing. They allow you to group parts of your regular expression to apply quantifiers or to treat multiple characters as a single unit. Additionally, parentheses capture the matched content, making it accessible for later use, such as when extracting data or in backreferences. 

Examples of Regular Expressions 

Here are some prominent examples of regular expressions: 

1) Matching Email Addresses: The Regex pattern for a basic email address can look like this: [w.-]+@w+.w+. This pattern matches email addresses like “[email protected]” 

2) Matching URLs: To match URLs, you may use a pattern like (https?|ftp)://S+. This matches URLs starting with “https,” “http,” or “ftp.” 

3) Matching Dates: d{2}/d{2}/d{4} could be a Regex pattern for matching dates in the format “mm/dd/yyyy”. 

4) Extracting Phone Numbers: To extract phone numbers in the format “+44 111-2222,” you can use the pattern (d{3}) d{3}-d{4}. 

5) Finding HTML Tags: To locate HTML tags in a text, you can use a pattern like < [^>]+ >. it matches any HTML tag, including < div >, < p >, etc.