What Are Regular Expressions?

Regular expressions facilitate text validation across various inputs without the need to explicitly define each one. That definition might sound perplexing, but it will become clearer through an example, I assure you!

Consider a scenario where a form requests a user's email address. To determine if the input is valid, we can verify the presence of an "@" symbol. However, checking every possible email configuration is impractical, which is where Regex comes into play.

Practical Application of Regex

Before delving into specific expressions, let's look at a basic example of how to implement Regex in your code.

Initially, a user submits data via a form, and this value is then stored. We compare this input against a specified regex pattern. If the input aligns with the criteria, the validation succeeds; if not, it is rejected.

In code, this logic can be represented as follows (using PHP syntax, but applicable across various programming languages):

if ($input === $pattern) {

return true;

} else {

return false;

}

Creating Expressions

For the examples below, we will use the variable $pattern to represent the regex expression and $input for the strings that need validation. Comments will indicate whether the input matches the pattern.

• [aeiou] : Matches any character within the brackets

  • $pattern = "[bcd]";
  • $input1 = "cat"; // Matches the pattern
  • $input2 = "mat"; // Does not match the pattern

• [A-Z] : Matches any uppercase letter from A to Z

  • $pattern = "[A-Z]";
  • $input1 = "Cat"; // Matches the pattern
  • $input2 = "mat"; // Does not match

• [^b-g] : Matches any character not between b and g

  • $pattern = "[^t]";
  • $input1 = "abc"; // Matches the pattern
  • $input2 = "ttt"; // Does not match

• d : Matches any digit

  • $pattern = "[d]";
  • $input1 = "f1nd"; // Matches the pattern
  • $input2 = "find"; // Does not match

• s : Matches any whitespace character

  • $pattern = "[s]";
  • $input1 = "cat and dog"; // Matches the pattern
  • $input2 = "catanddog"; // Does not match

• w : Matches any word character

  • $pattern = "[w]";
  • $input1 = "cat!"; // Matches the pattern
  • $input2 = "@@@"; // Does not match

• n* : Matches zero or more occurrences

  • $pattern = "colo[u]r";
  • $input1 = "color"; // Matches the pattern
  • $input2 = "colour"; // Also matches

• n+ : Matches one or more occurrences

  • $pattern = "[o]+";
  • $input1 = "cal"; // Does not match
  • $input2 = "col"; // Matches
  • $input3 = "cooool"; // Matches

• n? : Matches zero or one occurrence

  • $pattern = "c[a]?t";
  • $input1 = "cat"; // Matches
  • $input2 = "caat"; // Does not match

• n{x} : Matches exactly n times

  • $pattern = "spe{2}d";
  • $input1 = "speed"; // Matches
  • $input2 = "speeed"; // Does not match

• n{x,y} : Matches between x and y times

  • $pattern = "com{1,2}a";
  • $input1 = "coma"; // Matches
  • $input2 = "comma"; // Matches
  • $input3 = "commmma"; // Does not match

• xxx$ : Ensures the expression ends with a specific phrase

  • $pattern = "dogs$";
  • $input1 = "I like dogs"; // Matches
  • $input2 = "I like dogs and cats"; // Does not match

• ^xxx : Ensures the entry starts with a specific phrase

  • $pattern = "^They";
  • $input1 = "They like dogs"; // Matches
  • $input2 = "I like dogs"; // Does not match

• (x|y) : Matches input containing either x or y

  • $pattern = "(cat|dog)";
  • $input1 = "I like cats"; // Matches
  • $input2 = "I like dogs"; // Also matches
  • $input3 = "I like cows"; // Does not match

Combining Expressions

Each of the expressions mentioned can be combined to create more complex criteria. For instance, to specify an email address, we need several interconnected conditions:

• A selection of letters, numbers, or the "_" symbol of varying lengths
• An "@" symbol
• A selection of letters for the email host provider
• A "." symbol
• A collection of characters for the domain

$pattern = "[w]*@[a-zA-z]*.[a-zA-Z]*";

$input1 = "[email protected]"; // Matches the pattern

$input2 = "simply.mail.com"; // Does not match

Practicing Your Skills

Though initially daunting, the best way to grasp your Regex expressions is through practice. If you, like me, enjoy tackling mini-puzzles and learn better through challenges rather than textbooks, consider exploring the following resources:

• RegexOne

  • This site offers a comprehensive guide on the rules mentioned, along with interactive exercises to test your understanding:
  • RegexOne — Learn Regular Expressions — Lesson 1: An Introduction, and the ABCs

• HackerRank

  • A fantastic platform for solving coding challenges and reinforcing skills. You earn points and improve your rank for each challenge completed, which adds an element of fun:
  • Solve Regex Code Challenges

• Regex Crossword

  • A simple yet satisfying concept where you fill out a crossword that meets surrounding Regex expressions. The difficulty increases as you progress:
  • Regex Crossword

I trust this overview has been helpful, and I encourage you to check out these resources to enhance your knowledge. Mastering Regex is essential for effectively managing user inputs and performing data validation, making it a worthy investment of your time. (This topic remains a personal learning journey, so feel free to comment if you notice anything I've overlooked!)

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Thanks for reading!