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.
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.
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.
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.
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 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.
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.
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.
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".
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.
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.
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.
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).
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.
