Regular expressions

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Regular Expressions (regex) are essentially a search engine for finding patterns in a text. While the syntax is a bit tricky to learn, regex will save tons of time and effort in the long run. Many of you are probably familiar with regex, even if only through the use of wildcards. Wildcard notation, such as *.html, matches to all html files in the given search directory. Regex takes this idea and expands on it dramatically, allowing for very complicated search patterns. A regular expression to find all html files in a given directory would be .*\.html$

Syntax

The following information pertains to the Perl 5 Regex engine. Different engines have slightly different syntax

Characters

Literals

The most basic regex is a literal character. A literal character, such as a matches a in the string alex. However, in a string such as adam, it will only match the first a, before the 'd', unless you tell the regex engine otherwise. Most text editors that have a 'find' function, also have a 'find next' function.

Similarly, a regex search for hat in the string Blackhat Academy will return 'hat' from the end of the first word. This is merely a string of literal characters, and the regex engine handles it the same way as it handles a single literal character.

Metacharacters

Regex wouldn't really be that useful if we just wanted to match literal strings of text. That's where special characters come in to play. Each metacharacter has it's own use, and in order to use any of them as literal characters, you need to escape them using the \ (backslash) key.

There are 11 metacharacters we'll use:

• Opening square bracket [
• Backslash \
• Caret ^
• Dollar Sign $
• Dot .
• Pipe |
• Question mark ?
• Asterik *
• Plus +
• Open round bracket (
• Close round bracket )

Take 1+1=2 for example. The correct regex to match this string is 1\+1=2. It's important to note that 1+1=2 is still valid regex, but it wouldn't match 1+1=2. Instead, it would match 111=2 in the string 111=25*4+11. This is the a great example of how to properly escape special characters when you want to find their literal character counterpart, but how it might not always throw an error, even if you don't escape your special characters.

Non-Printable

Non-printable characters are any ASCII or ANSI character not represented in the standard A-z0-9 character set. Popular non-printable characters include the tab character, represented by \t or \x09, the carriage return (\r or \x0D), and the new line (\n or \0A). These are by no means the only non printable characters you can use in regular expressions. In fact, you can use any ASCII or ANSI character code for the character set you are working with.

Being able to use the ASCII character codes means that even if you're trying to match a character that isn't on your keyboard, you can still easily match it.

Protip: Windows uses \r\n to terminate lines, while UNIX derivatives terminate lines with just \n

Character Classes (Sets)

Character classes, put simply, allow regex engines to match only one out of several provided characters. A character class is denoted by square brackets around the character variations that you're looking for. For instance, if you want to find all instances of the word grey, both American and British, in a document, you could search for gr[ae]y. This would match either gray or grey. It will not, however, match graay or graey.

You can also use hyphens inside a character class to represent a range of characters, such as [0-9] to represent any digit. Similarly, [a-z] would match any single, lowercase letter, and [A-Z] would match any single, uppercase letter. More importantly, perhaps, is that you can chain these together. For instance, if I wanted to match any AlphaNumeric character, I could search for [a-zA-Z]. If I wanted to find any single hexadecimal digit, I could search for [a-fA-F0-9].

Negated Character Classes

Negating a character class is done by typing a caret ^ after the opening bracket. Negating a character class means it will match any character that's 'not' in the defined character class. It's important to note that even negated character classes have to match some character. A regular expression foo[^b] does not mean "match any instance of foo not followed by b," but rather "match any instance of foo followed by any character that isn't a b.

Tools

Utilities

• grep
• grepWin
• RegexPal

Programming Languages

• Gnulib
• Java
• JavaScript
• .NET
• Perl
• PHP
• PowerShell
• Python
• Ruby

Databases

• MySQL
• Oracle
• PostgreSQL