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Regex Tutorial Table of Contents

This regular expression tutorial teaches you every aspect of regular expressions. Each topic assumes you have read and understood all previous topics. So if you are new to regular expressions, I recommend you read the topics in the order presented.

Introduction

The introduction indicates the scope of the tutorial and which regex flavors will be discussed. It also introduces basic terminology.

Literal Characters and Special Characters

The simplest regex consists of only literal characters. Certain characters have special meanings in a regex and have to be escaped. Escaping rules may get a bit complicated when using regexes in software source code. How to enter non-printable characters.

How a Regex Engine Works Internally

First look at the internals of the regular expression engine's internals. Later topics will build on this information. Knowing the engine's internals will greatly help you to craft regexes that match what you intended, and not match what you do not want.

Character Classes or Character Sets

A character class or character set matches a single character out of several possible characters, consisting of individual characters and/or ranges of characters. A negated character class matches a single character not in the character class. Shorthand character classes allow you to use common sets quickly.

The Dot

The dot matches any character, though usually not line break characters unless you change an option.

Start of String and End of String Anchors

Anchors are zero-width. They do not match any characters, but rather a position. The caret and the dollar sign match at the start and the end of the string. Depending on your regex flavor and its options, they can match at the start and the end of a line as well.

Word Boundaries

Word boundaries are like anchors, but match at the start of a word and the end of a word. However, most regex flavors define the concept of a "word" differently than your English teacher in grade school.

Alternation

By separating different sub-regexes with vertical bars, you can tell the regex engine to attempt them from left to right, and return success as soon as one of them can be matched.

Optional Items

Putting a question mark after an item tells the regex engine to match the item if possible, but continue anyway (rather than admit defeat) if it cannot be matched.

Repetition Using Various Quantifiers

Three styles of operators, the star, the plus, and curly braces, allow you to repeat an item zero or more times, once or more, or an arbitrary number of times. It is important to understand that these quantifiers are "greedy" by default, unless you explicitly make them "lazy".

Grouping and Backreferences

By placing round brackets around part of the regex, you tell the engine to treat that part as a single item when applying operators such as quantifiers. With round brackets, you can also create backreferences that allow you to reuse the text matched by part of the regex inside the regular expression, or later in the replacement text of a search and replace operation. Backreferences are also very useful for extracting parts from a string in a programming language.

Unicode Characters and Properties

If your regular expression flavor supports Unicode, then you can use special Unicode regex tokens to match specific Unicode characters, or to match any character that has a certain Unicode property or is part of a particular Unicode script or block.

Mode Modifiers

Change matching modes such as "case insensitive" for specific parts of the regular expression.

Atomic Grouping and Possessive Quantifiers

Nested quantifiers can cause an exponentially increasing amount of backtracking that brings the regex engine to a grinding halt. Atomic grouping and possessive quantifiers provide a solution.

Lookaround with Zero-Width Assertions, part 1 and part 2

Lookahead and lookbehind (collectively lookaround) are zero-width. With positive lookaround, you can specify multiple requirements (sub-regexes) to be applied to the same part of the string. With negative lookaround, you can invert the result of a regex match (i.e. match something that does not match something else).

Continuing from The Previous Match Attempt

Forcing a regex match to start at the end of a previous match provides an efficient way to parse text data.

Combining Positive and Negative Lookaround with Conditionals

A conditional is a special construct that will first evaluate a lookaround, and then execute one sub-regex if the lookaround succeeds, and another sub-regex if the lookaround fails.

XML Character Classes

XML Schema regular expressions support four shorthand character classes to match XML names. They also introduce a handy feature called "character class subtraction", which is now also available in the JGsoft and .NET regex engines.

POSIX Bracket Expressions

If you are using a POSIX-compliant regular expression engine, you can use POSIX bracket expressions to match locale-dependent characters.

Adding Comments

Some regex flavors allow you to add comments to make complex regular expressions easier to understand.

Free-Spacing Mode

Splitting a regular expression into multiple lines, adding comments and whitespace, makes it even more readable.

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