Regular expressions are classified into two types: basic regular expressions (BRE) and extended regular expressions (ERE ). Metacharacters of the basic regular expression: 1 ,.: match any single character 2, []: match a single character within the specified range 3, [^]: match a single character out of the specified range 4 ,*: match the characters 5 ,\? : Match the first 0 or 1 6 times
Regular expressions are classified into two types: basic regular expressions (BRE) and extended regular expressions (ERE ).
The metacharacters of the basic regular expression:
1.. match any single character
2. []: match a single character in the specified range
3. [^]: match a single character out of the specified range
4. *: match any number of characters above it
5 ,\? : Match the first 0 or 1 times
6. \ +: match the first 1 or multiple times
7. \ {m \}: Exact Match m times
8. \ {m, n \}: match at least m times, at most n times
9. *: match any character of any length
10. ^: The first line is anchored,
11. $: anchor at the end of the line
12. \ <: specifies the beginning of a word (\ B can also be used)
13. \>: The end of the word is anchored (\ B can also be used)
14. ^ $: Match blank rows
15. \ (\): used together with \ n (n is a number) to match the reproduction times of a single or multiple character combinations.
II. extend the metacharacters of the regular expression:
1.. match any single character
2. []: match a single character in the specified range
3. [^]: match a single character out of the specified range
4. *: match any number of characters above it
5 ,? : Match the first 0 or 1 times
6. +: match the first 1 or multiple times
7. {m}: Exact Match m times
8. {m, n}: match at least m times, up to n times
9. *: match any character of any length
10. ^: The first line is anchored,
11. $: anchor at the end of the line
12. \ <: specifies the beginning of a word (\ B can also be used)
13. \>: The end of the word is anchored (\ B can also be used)
14. ^ $: Match blank rows
15. (): used together with \ n (n is a number) to match the reproduction times of a single or multiple character combinations.
16. |: Match | character before or after a symbol
3. commands that can match regular expressions include: (here we only use the grep command)
1. grep: grep uses a basic regular expression to match text rows.
Format: grep [OPTIONS] PATTERN [FILE...]
Grep [OPTIONS] [-e PATTERN |-f FILE] [FILE...]
Option:
-E: use an extended regular expression for matching (equivalent to egrep)
-F: equivalent to fgrep
-I: Case sensitivity is ignored during matching.
-V: displays rows in unmatched mode.
-O: only the matched rows are displayed.
-Q: Silent mode
-- Color = auto: highlight matched characters
IV. example:
1. display the rows ending with bash in the/etc/passwd file;
2. display two or three digits in the/etc/passwd file;
3. display the lines that end with 'listen' followed by 0, 1, or multiple white spaces in the 'netstat-tan' command results
4. add bash, testbash, basher, and nologin users (the shell of nologin users is/sbin/nologin), and find the rows in the/etc/passwd file with the same username as the shell name;
5. display the default shell and UID of the root, centos, or user1 user on the current system;
6. find a word in the/etc/rc. d/init. d/functions file (the word can contain underscores in the middle) followed by a group of parentheses.
7. use echo to output a path, and then egrep finds the base name of the path;
Further: use egrep to retrieve its directory name
8. find the number between 1-in the ifconfig command execution result;