Text Search grep Knowledge points Summary

Text Search tool: grep, Egrep
Filters the target file according to user-specified mode, displaying the rows to which the pattern matches
grep [OPTION] ... ' PATTERN ' FILE ...
--color

The difficulty lies in how the pattern is written, and the pattern is mainly the application of regular expressions
Regular expression: A pattern written by a class of characters, where some characters do not represent the literal meaning of a character,
But a function of control or wildcard.
Metacharacters: *,? etc
Two categories:
Basic Regular Expressions
Extending regular Expressions

Basic Regular Expressions:
Character Matching:
.: Matches any single character
For example: grep--color ' r. T '/etc/passwd
Match to R start, T end, two-character line between

[]: matches any single character in the specified collection
[[:d igit:]],[0-9]
[[: Lower:]],[a-z]
[[: Upper:]],[a-z]
[[: Alpha:]],[a-za-z]
[[: Alnum:]],[0-9a-za-z]
[[: Space:]]
[[:p UNCT:]]
For example: grep--color ' abcd[[:d igit:]][[:d igit:]][0-9] ' test
Matches a line that starts with ABCD, followed by three digits.

[^]: matches any single character outside the specified set
For example: grep--color ' abcd[^[:d igit:]] ' test
grep--color ' abcd[^0-9] ' test
Matches a line that begins with ABCD and is not a number behind

Number of matches: used to limit the number of occurrences of a character immediately preceding it
*: Match any of its preceding characters, 0,1 or multiple times
For example: grep ' X*y '
Xy,xxy,xxxy,y

\?: matches the preceding character 0 or 1 times, where \ is the escape character, which is the transfer?
For example: grep ' X\?y '
Xy,xxy,y,xxxy,aby for XXY, it does match to XY, and the first X is ignored.

\+: Matches the preceding character appearing at least once
For example: grep ' X\+y '
Xy,xxy,xxxy

\{m\}: Matches the preceding character m times
For example: grep ' X\{2\}y '
Xxy,xxxy

\{m,n\}: Matches the preceding character at least m times, up to N times
For example: grep ' X\{2,4\}y '
XXY, Xxxy
grep ' X\{2,\}y '
Xxy,xxxy

. *: Matches any character of any length

Location anchoring:
^: Anchor at the beginning of the line
Write on the leftmost side of the pattern
For example: grep--color ' ^ABCD '/etc/passwd
Match lines that begin with ABCD

$: End of line anchoring
Write at the far right of the pattern
For example: grep--color ' [0-9]$ '/etc/passwd
Match lines that end with a number

^$: Blank Line
For example: grep--color ' ^$ '/etc/passwd
Match Blank Lines

\&lt: initial anchor, \b, \ for escape character
To the left of the word pattern that you are looking for, \<char
For example: grep--color ' \<r '/etc/passwd
Match words that begin with R

\>: Ending anchor, \b,\ as escape character
The right side of the word pattern that you want to find now,char\>
For example: grep--color ' tor\> '/etc/passwd
Match words ending with Tor

\<pattern\>: Match words
For example: grep--color ' \<root\> '/etc/passwd
Match Word Root

Group:
\(\)
Back reference: In the pattern, if you use \ (\) to implement the grouping, in the examination of a line of text,
If the \ (\) pattern matches a certain content, the pattern behind this content can be referenced;
\1,\2,\3
Pattern from left to right, referencing the # opening parenthesis and matching the pattern between the closing parenthesis and its match

For example: grep--color ' ab\{1,\}y ' test
At least once b between a and Y
grep--color ' \ (ab\) \{1,\}y ' test
Before y at least once AB
grep--color ' \ (ab\) \{1,\}y\1 ' test
Before y at least once AB, and the first group is referenced behind
grep--color ' \ (ab\) \{1,\}y\2 ' test
Before y at least once AB, and a second group behind

grep options:
-V: Reverse selection, which shows lines outside the pattern match
Example: Grep-v--color ' ABCD ' test
Show lines outside of ABCD
-O: Show only what matches to
-I: Ignore character case
-N: Display line numbers when matched
-E: Using extended regular expressions
-A #: Match to the bottom # line
Example: Grep-a 2--color ' ABCD ' test
Display matching ABCD lines and their bottom two lines, total three rows
-B #: Match to the top # line
-C #: Match to the top and bottom # lines

Egrep and extended Regular expressions
Extending the meta-character of a regular expression
Character Matching:
.
[]
[^]

Match number limit:
*
?: match its preceding character 0 or 1 times
+: Match its preceding character at least 1 times
{m}: matches its preceding character m times
{M,n}:{m,},{0,n}

Anchoring:
^
$
\<, \>: \b

Group:
()
Support for back reference: \1, \2 ...

Or:
A|b:a or B
AB|CD:
Example: Egrep--color ' ab|c ' test
grep--color ' ab\|c ' test
Match a line containing AB or C
grep--color ' a\ (b\|c\) ' Test
Match a line containing AB or AC

Example: Grep-e ' PATTERN ' FILE ...
Egrep ' PATTERN ' FILE ...

Text Search grep Knowledge points Summary