grep and regular expressions

A. grep concept

grep, the search for regular expressions, which means that grep is simply used to search for matching characters.

Second, the regular expression:

(1) Several repetitive actions followed by regular expressions

- ? Match at most once

-* Match 0 or more times

-+ matches more than once

-{n} matches n times

-{n,} matches at least n times

-{, m} matches up to M times

-{n,m} matches n to M times

(2) commonly used regular expression chestnuts:

^ Beginning of the line

$ End of line

[] Match any one that appears

^user$ only contains user

[tty]$ line ending with TTY

^ [^ l] Exclude directory list of associated directories

[. * 0] 0 before or after adding any character

[^ $] blank line

[^. * $] matches any string in a row

^ . . . . . . $ includes 6-character lines

^ Q start line with ^ q

^ . $ only one character line

^\. [0-9] [0-9] lines starting with a period and two digits

[0-9] \ {2 \}-[0-9] \ {2 \}-[0-9] \ {4 \} date format d d-m m-y y y y

[0-9] \ {3 \} \. [0-9] \ {3 \} \. [0-9] \ {3 \} \. [0-9] \ {3 \} I p address format

[a-za-z]* matches all words

Iii. Common grep options

(1)-C outputs only the count of matching rows.

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(2)-I is case-insensitive (only for single-character).

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(3)-n displays matching lines and line numbers.

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(4)-S does not display an error message that does not exist or has no matching text.

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(5)-V displays all lines that do not contain matching text.

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File1 is a file copy, that is, the CP file File1

(6)-L only output file names that contain matching characters when querying multiple files.

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(7)-H does not display the file name when querying multiple files.

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(8) Multiple filtration:

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(9) Use grep to match "and" or "or" mode

The grep command plus-e parameter indicates that extended pattern matching is allowed.

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-W searches the entire word, not the partial string in the word.

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(one)-A displays the n rows after the matching row

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-B Displays the n rows before matching rows

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-c display n rows before and after matching rows

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-R recursively searches all files

usage:$ grep-r "file" *

-O displays only matched strings (useful when using regular expressions)

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-B Displays the matching location

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Note: The above output shows not the position within the line, but the byte position in the entire file

--color the searched characters are color-recognizable and easy to identify

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-Q silent mode, not output, often used in shell scripts

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Iv. using regular Expressions in grep

(1). Match any single character

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(2) * matches the previous character any time (can be 0 times)

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(3). * matches any character of any length (greedy mode)

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(4) [] any single character within the specified range

[^] characters outside the specified range

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(5) X\{m,n\} X appears greater than m times, less than n times

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(6) X\{m,\} X appears greater than M, no limit

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(7)? Matches the preceding character 0 times or once (-E, using an extended expression)

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(8) Anchor ^ anchor start (line match)

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(9) $ Anchor Line end (line end matching)

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(10) ^$ blank line

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grep and regular expressions