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  grep regular expression
  Add Date : 2017-01-08      
  Regular expressions are a notation for identifying text mode. Linux main proceedings dealing with regular expressions is grep. grep searches and regular expression matching rows, and the results delivered to the standard output.

1. grep pattern matching

grep following manner acceptable options and arguments (where, regex regular expression)

grep [options] regex [files]
Where options are mainly in the following table:

Option Meaning Functional Description
-i ignore case ignore case
-v invert match does not match match
-l file-with-match output matching file names
-L File-without-match output does not match the file name
-c count the number of matching output (number of rows)
-n number output matching lines simultaneously preceded by the file name and line number in the file name
-h no-filename suppress the output file name

2. Special characters

Symbol Meaning Example
^ Start tag "^ abc" meet the case abc, abcd
Non ^ (in the []) "[^ abc]" satisfy example: ddd, mpd
$ End tag "abc $" satisfy example abc, mmabc
Arbitrary character "a.c" meet the case abc, fapcc
< Matching words beginning "< abc" meet the case abc, abcd
> Matching words end "abc>" meet the case abc, pmrabc
| Or "AAA | BBB" satisfy example AAA, BBBpp
3. Scope

Symbol Meaning Example
? Matches the preceding character 0 or 1 time "abc?" Examples met ab, mabcd
* Matches the preceding character >=0 second example "abc *" satisfied abbb, abcdk
+ Matches the preceding character >=1 times "abc +" satisfy example abcd, abcccdd
{} {M}, {m, n}, {m,}, {, n} Match the previous character respectively m times, m to n times, >=m times, <=n times "abc {3,5}" satisfy example abcccc, abcccccc
[] [] If not within range, choose one; the range, then choose one "m [abc] p" to meet the range of examples acpd; m [1-9] p satisfy example m8pp
() On all the elements of the candidate (inside), with | separated by "a (1 | 2 | 3) bc" satisfy example a1bc, mba3bcd
Note: {} need to be transferred in Zheng expressions, and {} () is not required.

4. Standard character class

Character Class Definition
[: Alnum:] letters and numbers, and [A-Za-z0-9] equivalent
[: Word:] [: alnum:] plus underscore _
[: Alpa:] letter, and [A-Za-z] Equivalent
[: Digit:] numbers, and [0-9] equivalent
[: Xdigit:] hexadecimal characters, and [0-9A-Fa-f equivalent]
[: Blank:] spaces and tabs
[: Graph:] visible character, expanding by 33 to 126
[: Lower:] Lowercase letters
[: Upper:] capital letters
[: Print:] printable characters
[: Space:] whitespace characters, equivalent to [trnvf]
[: Punct:] Punctuation
[: Cntrl:] ASCII control codes, including characters from 0 to 31 and 127
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