^

start of string

$

end of string

Character Classes

\s

white space including spaces, tabs, and line breaks

\S

not white space

\d

digit, same as [0-9]

\D

not digit

\w

word

\W

not word

Quantifiers

*

0 or more of preceding characters

+

1 or more of preceding characters

?

0 or 1 of preceding character

Escape Character

\

put before a metacharacter to search for that actual character

Metacharacters

^ [ . $ { * ( \ + ) | ? < >

must be used with the Escape Character if you are searching specifically for it

Special characters

\n

new line

\t

tab

Groups and Ranges

.

any character except new line (\n)

(a|b)

a or b

()

group

[abc]

a or b or c

[^abc]

not a or b or c

[a-q]

letter between a and q

[A-Q]

upper case letter between A and Q

[0-7]

digit between 0 and 7

Pattern modifiers

i

case insensitive

Other helpers

.*

swallows text between 2 words

\s+

guarantees minimum of 1 whitespace between 2 words

\s*

guarantees 0 or more whitespace between 2 words

^\s*

beginning of line with zero or more whitespace

\s+.*

swallows all text and white space between 2 words

\d+

guarantees minimum of 1 number between 2 words

<\w>

identical to <[a-zA-Z0-(_]>

([A-Za-z0-9]+)

Letters, numbers, hyphens

Find the line beginning with sshd. Then, using a colon : as delimiter, check that the value four ":" over is equal to 99999.

^sshd:[^:]*:[^:]*:[^:]*:99999:

Verify that PASS_MAX_DAYS exists starting in position 1 and a value after it is ⇐ 90.

^\s*PASS_MAX_DAYS\s+([0-9]|[1-8][0-9]|90)

Verify that ROOTPW (in any case) exists on an uncommented line.

/^[#]*ROOTPW/i

Verify that line in file starts with size and the value after is ⇐ 4096k.

^\s*size\s+(409[0-6]|40[0-8][0-9]|[123][09]{3}|\d{1,3})k

Find line with string restrict 127.0.0.1 that starts in position `1 to ensure it is not commented out.

^\s*restrict\s+127\.0\.0\.1

Find an uncommented line that contains /home. There will be additional text before the desired string.

^[#]*\/home