Regular Expressions

Reference

Topic Title One

Basic Regular Expressions
Extended Regular Expressions
Perl Compatable Regular Expressions
Bash: grep & sed

Glossary

Where "x" means any alphabetic character Where n or m means any numeric character etc.

Metacharacters

^x

"caret": Only looks at the beginning of the target string for x

x$

"dollar": Only looks at the end of the target string for x

x.y

"period": Looks for a string with any character in this position, but there must be a character

x?y

"question mark": Looks for a string with any character in this position or no character in this position

x*

"asterick": Looks for a string where the character x appears any number of times or no times

x+

"plus": Looks for a string where the character x appears one or more times

[abc]

"square brackets": Looks for a string that contains any of the characters inside the square brackets ("expression") any number of times

[a-c]

"square brackets": Looks for a string that contains the first character a or any other character until the second character c based on dictionary ordering Warning: Depending on your computer location settings what characters are included can differ!

[^abc]

"square brackets": Looks for a string that does not contain any of the characters abc within the square brackets

x{n}

"curly braces": Looks for a string where the preceding character x appears exactly n times

x{x,m}

"curly braces": Looks for a string where the preceding character x appears at least n but not more than m times (n to m inclusive)

x{n,}

"curly braces": Looks for a string where the preceding character x appears at least n times (great than or equal to n)

(x)

"parantheses": Group parts of a search string together into a subexpression or to make the result of the string inside the parantheses a variable which you can later refer back to note: Can be nested infinitely

x|y

"pipe": Looks for a string that matches the first character x and if the first is not matched then will look for a string that matches the second character

Character and Positional Abbreviations

Note that these are implemented in most languages, but not absolutely all. Search "Perl Compatable Regular Expressions" (PCRE) for more information. There also might be more of these abbreviations in your chosen language.

\n

Represents a newline (line break)

\r

Represents a carriage return (return or enter)

\t

Represents a horizontal tab

\d

Represents any character in the range 0 - 9 (zero to nine inclusive)

\D

Represents any characters not in the range 0 - 9 (zero to nine inclusive)

\s

Represents a whitespace character (space or tab)

\S

Represents any character that is not a whitespace character (space or tab)

\w

Represents any character in the range 0 - 9, A - Z, a - z (all alphanumeric characters)

\W

Represents any character that is not in the range 0 - 9, A - Z, a - z (all alphanumeric characters)

\bx

Represents the beginning of a word. Looks for a string where the following character x appears at the beginning of a word

x\b

Represents the end of a word. Looks for a string where the preceding character x appears at the end of a word

\Bx

Represents any position in a string that is not the beginning or the end. Looks for a string where the following character x does not appear at the beginning of a word

x\B