grep and regular expressions

grep: Search for text according to pattern, and display text that conforms to the pattern

Pattern: A matching condition in which the literal character wildcards regular a combination of the meta-characters of the expression.

Single quotation marks: Strong references

Double quotation marks: weak references

-I: Ignore case

--color: Match to result plus color

Alias grep= ' grep--color '

-V: Show rows that are not matched by the pattern

-O: Displays only the string to which the pattern matches

-E: Using extended regular expressions

-A N: Shows the rows to match and the N rows following the row

-B N: Shows the rows to match and the N rows in front of the row

-C N: Shows the rows to match and the N rows after the line

*: Any character of any length

?: any single character

Regular Expressions: REGular expreesion, REGEXP

Metacharacters

. : Match any single of characters

[]: matches any single character within the specified range

[^]: matches any single character outside the specified range

[:d Igit:], [: Lower:], [: Upper:],[:space:]

Number of matches (greedy mode):

*: Matches any of its preceding characters any time

A, B,ab,aab, ACB, ADB, AMNB

A*B:B, AB, AaB

A.*b:ab, AAB, ACB, ADB, AMNB

. *: Any character of any length

\?: Match its preceding character 1 or 0 times

\{m,n\}: Matches the preceding character at least m times, up to N times

\{0,3\}

Location anchoring:

^: Anchor the first line, any content after this character must appear at the beginning of the

$: Anchor line end, any content after this character must appear at the end of the row

^$: Blank Line

\< or \b: Any subsequent character must appear as a word header

\> or \b: Any character in front of it must appear as the tail of the word

\<root\> or \broot\b: Exact match

Group:

\(\)

\ (ab\) *: AB as a whole, appears one or more times

1\: Refers to the first opening parenthesis and the corresponding closing parenthesis to match the contents of the

2\:

Exercise: Analyze the characteristics of the first two lines in the following text in the/etc/inittab file, and write a pattern that can be precisely similar to the current one.

11:1:WAIT:/ETC/RC.D/RC 1

13:3:WAIT:/ETC/RC.D/RC 3

grep ' ^1\ ([0-9]\): 1\.* 1\$ '/etc/inittab

To extend the regular expression:

Character Matching:

.

[]

[^]

Number of matches:

*:

?:

+: Match its preceding character at least 1 times

{M,n}

Location anchoring:

^

$

\<

\>

Group:

(): Group

\1,\2,\3,.....

Or:

| : OR

C|cat: Match C or cat

(C|c) at: Match cat or cat

grep and regular expressions