Regular Expressions

Regular expressions are characters and meta-characters that are used to identify parts of text. Using meta-characters, pattern matching can be very specific or generalized. They are used to find and take actions on text. The following is not a comprehensive list, but does cover the majority of operations that are used when parsing logs/text/files.

Note: For quick testing/building of regular expressions sites such as RegExr.com can really help.

Specifying Position

These are used to specify a position within a string or line.

^

• Start of a line/string

$

• End of a line/string

\A

• Start of a string

\Z

• End of a string

Specifying Characters

These are used to specify a particular type of character.

.

• Any character except a newline

\c

• Control character

\d

• Digit character (e.g. 0-9)

\D

• Non-digit character

\n

• New line character

\O

• Octal digit

\r

• Carriage return character

\s

• Whitespace character (tab/space/etc)

\S

• Non-whitespace character

\t

• Tab character

\w

• Word

\W

• Non-word

\x

• Hexadecimal digit

Specifying POSIX Character Classes

These are alternative nomenclatures for specifying character types under the POSIX standard.

[:upper:]

• Uppercase characters [A-Z]

[:lower:]

• Lowercase characters [a-z]

[:digit:]

• Any digit character [0-9]

[:space:]

• Any space character (space/tab/etc)

[:alpha:]

• Any uppercase or lowercase alphabetical character [A-Za-z]

[:alnum:]

• Any uppercase, lowercase, or digit character [A-Za-z0-9]

[:punct:]

• Any punctuation character

[:xdigit:]

• Any hexadecimal digit

[:cntrl:]

• Any control character

Specifying Quantity

These are used to specify how many times the preceding pattern has to match. For example:

\d{3}-?\d{2}-?\d{4}

• Matches a Social Security Number (SSN) format either with or without dashes.

*

• Zero or more instances of the previous pattern (or single character)

+

• One or more instances of the previous pattern (or single character)

?

• Zero or one instance (only, not more than) of the previous pattern (or single character)

{NUMBER}

• Exactly NUMBER instances

{NUMBER,}

• NUMBER or more instances

{NUMBER_A, NUMBER_B}

• NUMBER_A to NUMBER_B instances

Specifying Logic

These are used to specify how matching occurs and are used to make more complex patterns

[ ]

• Specify a range

[A-M]

• Single character in the range inclusive between "A" and "M" (e.g. "A", "B", "C", "D", ... "K", "L", "M")

[1-4]

• Single digit in the range inclusive between 1 and 4 (e.g. 1, 2, 3, 4)

(A|B)

• Single character that is either "A" or "B"

[ABC]

• Single character that is either "A" or "B" or "C"

[^ABC]

• Single character that is not "A" and not "B" and not "C"