regular expressions reference

Character

Definition

Example

^

The pattern has to appear at the beginning of a string.

^cat matches any string that begins with cat

$

The pattern has to appear at the end of a string.

cat$ matches any string that ends with cat

.

Matches any character.

cat. matches catT and cat2 but not catty

[]

Bracket expression. Matches one of any characters enclosed.

gr[ae]y matches gray or grey

[^]

Negates a bracket expression. Matches one of any characters EXCEPT those enclosed.

1[^02] matches 13 but not 10 or 12

[-]

Range. Matches any characters within the range.

[1-9] matches any single digit EXCEPT 0

?

Preceeding item must match one or zero times.

colou?r matches color or colour but not colouur

+

Preceeding item must match one or more times.

be+ matches be or bee but not b

*

Preceeding item must match zero or more times.

be* matches b or be or beeeeeeeeee

()

Parentheses. Creates a substring or item that metacharacters can be applied to

a(bee)?t matches at or abeet but not abet

{n}

Bound. Specifies exact number of times for the preceeding item to match.

[0-9]{3} matches any three digits

{n,}

Bound. Specifies minimum number of times for the preceeding item to match.

[0-9]{3,} matches any three or more digits

{n,m}

Bound. Specifies minimum and maximum number of times for the preceeding item to match.

[0-9]{3,5} matches any three, four, or five digits

|

Alternation. One of the alternatives has to match. ‘or’ operater

July (first|1st|1) will match July 1st but not July 2

POSIX Character Classes

Character

Definition

Example

[:alnum:]

alphanumeric character

[[:alnum:]]{3} matches any three letters or numbers, like 7Ds

[:alpha:]

alphabetic character, any case

[[:alpha:]]{5} matches five alphabetic characters, any case, like aBcDe

[:blank:]

space and tab

[[:blank:]]{3,5} matches any three, four, or five spaces and tabs

[:digit:]

digits

[[:digit:]]{3,5} matches any three, four, or five digits, like 3, 05, 489

[:lower:]

lowercase alphabetics

[[:lower:]] matches a but not A

[:punct:]

punctuation characters

[[:punct:]] matches ! or . or , but not a or 3

[:space:]

all whitespace characters, including newline and carriage return

[[:space:]] matches any space, tab, newline, or carriage return

[:upper:]

uppercase alphabetics

[[:upper:]] matches A but not a

Perl-Style Metacharacters

Character

Definition

Example

//

Default delimiters for pattern

/colou?r/ matches color or colour

i

Append to pattern to specify a case insensitive match

/colou?r/i matches COLOR or Colour

\b

A word boundary, the spot between word (\w) and non-word (\W) characters

/\bfred\b/i matches Fred but not Alfred or Frederick

\B

A non-word boundary

/fred\B/i matches Frederick but not Fred

\d

A single digit character,this is equivalent to the class [0-9].

/a\db/i matches a2b but not acb

\D

A single non-digit character,this is equivalent to the class [^0-9].

/a\Db/i matches aCb but not a2b

\n

The newline character. (ASCII 10)

/\n/ matches a newline

\r

The carriage return character. (ASCII 13)

/\r/ matches a carriage return

\s

A single whitespace character,this is equivalent to the class [^ \t\n\r\f\v].

/a\sb/ matches a b but not ab

\S

A single non-whitespace character,this is equivalent to the class [^ \t\n\r\f\v].

/a\Sb/ matches a2b but not a b

\t

The tab character. (ASCII 9)

/\t/ matches a tab.

\w

A single word character – alphanumeric and underscore,this is equivalent to the class [a-zA-Z0-9_].

/\w/ matches 1 or _ but not ?,

\W

A single non-word character,this is equivalent to the class [^a-zA-Z0-9_].

/a\Wb/i matches a!b but not a2b