Shell text filtering (regular expressions) __ Regular Expressions

When you extract or filter text from a file or command output, you can use regular expressions (R E), which are collections of special or not very special string patterns.

^ matches only the beginning of the line
$ only match end of line
* Only one single character followed by *, matching 0 or more of this single character
[] matches only the characters in []. Can be a single character, or it can be a sequence of characters. can be used-to represent the range of character sequences within [], as in the case of [1-5] instead of [1 2 3 4 5]
\ is used only to mask the special meaning of a single meta character. Because sometimes in S H e l l some meta characters have
Special meaning. \ can make it lose its meaning
. Match just any single character
P a t e r n \ {n \} is used only to match the occurrences of p a t T e R N in the front. n is the number of times
p a t t e R n \ {n,\} M only has the same meaning, but the least number is n
p a t t e R n \ {n,m \} only has the same meaning, but p a t e r n occurs in times between N and M

Period "." Match single character
1). : matches any single ASCII character, either as a letter, or as a number.
2) For example:.. XC.. Match dexc1t, 23XCDF, etc.,. W. W.. W. Matching rwxrw-rw-
To match a string or a sequence of characters at the beginning of a line
1 ^: Allows matching characters or words at the beginning of a line.
2) Examples: ^.01 matching 0011cx4, c01sdf, ^d matching drwxr-xr-x, drw-r--r--, etc.
End of line with $ match string or character
1) $: matches a string or character at the end of the line, and the $ symbol is placed after the matching word.
2) Example: trouble$ matches all lines ending with the word trouble
^$ matches all blank lines

Use * to match a single character in a string or its repeating sequence (unlike the "*" in the file name substitution)
1) *: A single character followed by the *, matching 0 or more of this single character.
2) Example: Compu*t will match character U one or more times, that is, match computer computing Compuuute etc.
1033* can match 101333 10133 1013444 etc
3 The use of "*" in regular expressions can sometimes produce unexpected results.

Use \ Mask The meaning of a special character
1) \: Used to mask the special meaning of a meta character. Because sometimes the meta characters have special meanings in the shell. \ You can make it lose its meaning.
2) Example: matches all files ending with *.pas in a regular expression: \*\.pas$

Use [] to match a single character belonging to a range or collection
1) []: matches the characters within "[]". Can be a single character, or it can be a sequence of characters. You can use "-" to denote the range of character sequences within the parentheses "[]".
In the case of [1-5] instead of [12345]. You can use commas "," to delimit the characters within the parentheses "[]".
2 when "^" symbol when directly leaning on "[", means negative or mismatched brackets "[]" in the contents
3) Example: [0-9] matches any number, [A-z] matches any one lowercase letter; [0-9a-za-z] matches any letter or number;
[C,c]omputer matching computer and computer;[^a-za-z] matches any non-alphabetic character

Number of occurrences of the pattern result using ' \{\} '
1) pattern\{n\}: The pattern of matching patterns occurs n times.
2) pattern\{n,\}: Match pattern patterns appear at least n times.
3) pattern\{,m\}: Match pattern patterns appear up to M times.
4) Pattern\{n,m\}: Match pattern patterns occur in the case between N and M.
5) Example: a\{2\}b matching value is AaB
A\{2,\}b matching values can be AaB or Aaaaab, but cannot match AB
A\{2,4\}b matching values can be AAB, Aaab, Aaaab, but cannot match AB or AAAAAB, etc.
[0-9]\{4\}cx[0-9]\{4\} match number 4 times followed by CX, and finally a number 4 times
6 actually the real format is {n} {n} {, m} {n,m}, except that the ESACPE character "\" is applied to "{" and "}".

Examples of regular expressions that are often used
[Ss]igna[ll] matches words SignaL, SignaL, SignaL, SignaL
[Ss]igna[ll]\. Ditto, but with a period
^user$ contains only the rows of the USER
\. Line with a period
^d.. X.. X.. x directories that have executable permissions for users, user groups, and other users, group members
^[^L] List of file directories after the symbolic link file (that is, rows that do not start with "L")
[Yynn] Uppercase or lowercase y or n
^.*$ matches any string in the row
^......$ includes 6-character lines
[A-za-z] Any single letter
[a-z]* at least one lowercase letter
[^0-9\$] Non-numeric or dollar sign
[123] 1 to 3 of a number
\^q starts with ^q.
^.$ line with only one character
^\. [0-9] [0-9] A line starting with a period and two digits
[0-9]\{2\}-[0-9]\{2\}-[0-9]\{4\}
Date format dd-mm-yyyy
[0-9]\{3\}\. [0-9]\{3\}\. [0-9]\{3\}\. [0-9]\{3\} class IP address format

nnn.nnn.nnn.nnn
. * Match any number of characters