Linux-grep and regular expressions

The need for text lookup:
Grep,egrep,fgrep
grep: Searches the text according to the pattern, and displays the line of conformance to the pattern,
Pattern: A matching condition in which a literal character wildcards expressions a combination of the metacharacters of an expression
Format: grep [option] Pattern file ...
-I.
--color
-V: Shows rows that are not matched by a pattern
-O: Displays only strings that are matched to the pattern
-E: Using extended regular Expressions =egrep
-A N: After n rows of matching rows
-B N: before n rows matching rows
-C N: Matches the front and back n rows of a row
Compare the globbing you've learned before:
*，。 , [],[^]
Mans 7 Glob


Regular expression: Regular expression,regexp
Note: grep is a partial match of the entire row (the longest match), showing the entire row


Regular expression Categories:
1, basic REGEXP: Fundamental
2, Extended REGEXP: Expansion




Metacharacters
. Represents matching a single arbitrary character
[]: matches any single character within the specified range
[^]: matches any single character outside the specified range
Character set: [: space:][:d igit:],[:lower:] ...


Number of matches:
*: matches the preceding character any time
A*b
A,b,ab,aab




. *: Any character of any length
。 : matches the preceding character 1 or 0 times (typically with a translation character \. )
\{m,n\}: Matches the preceding character at least m times, at most n times


Position Anchor:
^: Anchor the beginning of the line, any content after this character must appear at the beginning of the line
$: Anchor line end, any content after this character must appear at the end of the row
Eg:grep ' ^a.*a$ ' Test.txtt #以a开头和结尾的行
^$: Blank Line


\< or \b: Any character that follows must appear as the header of the word
\> or \b: Any character preceding it must appear as the end of the word
Eg:\<root\&gt: The word must be root,rootroot or not.


Group:
\(\)
Eg:\ (ab\) *,ab can occur any time




Another important use of grouping is to implement a back-to reference:
\1: Referencing the first bracket grouping




Extended Regular Expression:
Character Matching:
.
[]
[^]


Number of matches:
*
?: no \
+: Matches the preceding character at least 1 times
{M,n}


Position Anchor:
^
$
\<
\>


Group:
(): Grouping
\1,\2,\3 ...


Or
|: Indicates or


Practice:
Match 1-255 of the numbers
Egrep ' \< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-9]) \> ' test.txt


Match 255.255.255.255 of a numeric string of this type
Egrep ' (\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-9]) \>\.) {3} (\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-9] \>\.) ' Test.txt


Why not be quoted later.


Fgrep: Regular expressions are not supported, but the execution speed is fast