Regular Expression format and usage

Use regular expressions in various formats and usage

Commands that use regular expressions for search and search are commonly used: grep (search), egrep (Extended grep), and fgrep.

Grep: Use regular expressions, text information, and other conditions to retrieve information in a file and display lines containing matching strings.

Format: grep [Options] 'pattern' filename

Options: Option (omitted)

Pattern: Regular Expression and text

Filename: File Name

Option:

-I: case-insensitive characters

-- Color: color the matched string.

-O: only matching strings are displayed"

-V: displays unmatched rows.

-A #: displays the matched rows and the following # Rows (total # + 1 rows)

-B #: display the matched rows and the above # Rows

-C #: display the matched rows and the above # Rows and above # Rows (total # + 1 rows)

Pattern:

'.': Indicates a single arbitrary character.

'*': Indicates matching any character before

'. *': Any character of any length

'[]': A line that matches any single character

[: Alpha:] [: digit:] [: alnum:] [: lower:] [: Upper:]

'[^]': Match any row other than this character

\ {M, N \}: match the first character at least m times, at most N times

\? : Match the first character 0 or 1 time

\ <: Match the line with the first character after the word \>: match the line with the last character before the word

\ <String \>: match the row where the word 'string' is located

^: Match the row whose first line is its second character $: match the row whose last line is its first character

^ $: Matches blank rows

^ String $: the row that matches the entire 'string'

\ (String \): Group

Example: ^ \ (ABC \) [[: Space:] \ 1 $: match the line with the content 'abc abc'

\ 1 indicates the first group in the expression, \ 2: indicates the second group, and so on

Extension expression

Format: egrep [Options] pattern [files...]

{M, n} matches the first character at least m times and at most N times

+ Match the first character at least once. Similar to {1 ,}

() Group

| Or B | boy indicates B or boy (B | boy) indicates boy or boy

Other usage is similar to grep

Fgrep does not support regular expressions.

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Regular Expression format and usage