regex

Getting to Know Regex: What Is a Regular Expression?

Did you know that over 250,000 people in the United States do computer programming?

If you too aspire to be a competent coder in the future, having a strong foundation is a good starting point. That’s why studying languages like C#, Java, and more are important. But when programming in C, you might ask, “What is a Regular Expression?”

Don’t feel mystified yet.

With this guide, you’ll know what RegEx is and how important it is for your growth as a programmer. Read on and find out more.

What’s the Use of RegEx?

Regular Expressions (RegEx) enables coders to discover patterns in text strings. For example, matching a valid email address or password uses RegEx. One of its capabilities is allowing users to define search criteria to fit a pattern to their needs.

RegEx in C is like its own language, and all forms in the world use its patterns at varying levels. Each of these has similar functions, but they’re distinct. As a programmer, you’ll have lots of control when deciding on a pattern to search for.

But RegEx’s usage isn’t limited to forms, despite the latter’s necessity to the majority of the billions of websites in the world. If you have dynamic data using a text or string format, checking it against specific patterns can validate it against a database. With dynamic strings, you need RegEx to confirm whether it has your desired patterns.

Learning regular expressions search with one language allows you to do it in others. It matters not whether it’s JavaScript, Python, PHP, Perl, or Java. RegEx is a transferable skill that can add to your credibility as a software or web engineer.

You can also check out https://setapp.com/how-to/regex-quick-start if you want to learn more about Regular Expressions.

Understanding Literals

Literals are the most basic building block used in RegEx, also known as characters. A RegEx pattern with characters will mostly have no special meaning. It means they simply match each other, which means a single string matches this pattern, identical in every way possible.

Escaping Literal Characters

Some characters have special meaning, meaning you should use a backslash if you mean to represent themselves. Some of these include the caret (^) symbol, the dollar sign ($), and the asterisk (*). These can function as boundary matchers and quantifiers.

Non-Printable Characters

When using RegEx, you might encounter non-printable characters like the newline and tab character. When it becomes necessary to reference them, the best method is to use the right escape sequences. For example, “\t” represents tab while “\n” is for the newline.

Unicode Characters

Some characters are better when expressed using their Unicode index. In other times, you might match characters that you simply can’t type. These include control characters like ASCII ESC, VT, NUL, and more.

In some cases, the programming language you use can’t support pattern recognition for specific characters. For example, the G-clef symbol and emojis are outside the supported verbatim. These are characters outside the Basic Multilingual Plane (BMP).

Most RegEx engines use the escape syntax \uHexIndex to match characters using its Unicode Index. These are especially effective in Java, Python, Ruby, and JavaScript. Take note, Unicode support and escape syntax will differ depending on the engine used.

If you’re matching technical symbols, emojis, or musical symbols, study your RegEx engine. Most will have documentation available to support your specific use-case.

Escaping Pattern Components

In some cases, patterns need consecutive escaped characters as literals. For example, when matching a string +???+, the pattern should be something like \+\?\?\?\+. To escape every character as literal makes reading and understanding it more difficult.

Depending on your preferred RegEx engine, you’ll have methods to start and finish a section of your pattern as literal. Check your documentation for more details, but Java and Perl have this feature. For example, they enclose characters with literal interpretation between \Q and \E.

Escaping pattern parts is useful when you build it from parts. Some of these need literal interpretations, such as user-given search terms. If your chosen RegEx engine has no such features, its ecosystem often gives functions to escape all characters with designated special meanings from pattern strings.

Character Classes

You use these for defining a list of allowed characters. You put these in square brackets, and since it has alternatives, it will match exactly one character. For example, a pattern with [ab][cd] will match four strings—ac, ad, bc, and bd.

Using this example, it won’t match ab. The first character will match, but the second character must be something from the second set. It means it must either be c or d.

Ranges

Listing all possible characters using a character class often gets tedious and prone to errors. That’s why if you’re listing consecutive characters, use the dash operator to include them in a character class as ranges. For example, a [0123456789] class is simpler with [0-9] instead.

Characters use a numeric index to determine order using the Unicode index. When working with numbers, basic punctuation, and Latin characters, look at ASCII. This is a subset of Unicode, smaller and more historical.

Negations

In some cases, it’s better to define character classes that match most characters, except for the ones you define. When character classes start with a ^, it inverts the listed characters set. For example, if you’re allowing any character except digits and underscore, you use this statement: [^0-9_].

Predefined Character Classes

Some character classes see lots of use that developers use shorthand notations to define them. For example, the character class [0-9] is so common that they made a mnemonic notation to make it more convenient. Instead of typing that character class, you can use \d instead.

Most engines have an exhaustive list of predefined character classes. They match specific elements of the Unicode standard, alphabets, punctuations, and more. Its drawback is these aren’t portable.

Learn What Is a Regular Expression Today

This is the tip of the RegEx iceberg. It’s a good starting point if you want to know what is a regular expression and how it works.

That was a lot to soak but we hope you found this guide informative. If you did and you want more, read our other posts and discover even more tips and tricks today!