Grep and Regular Expression

1. grep basic syntax

The role of grep is to search. Unlike find, grep is as follows:

Find is used to search for directories and file names.

Grep is basically used to retrieve file content, strings, etc.

       grep [OPTIONS] PATTERN [FILE...]       grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

Options: (the most common)

-A search by text file

-C: calculates the number of matching rows.

-I case-insensitive

-N by the way, the output row number

-V reverse selection, that is, finding rows without a search string

Pattern:

(1) string to be searched

(2) Basic Regular Expression

(3) extended regular expression (The-e parameter needs to be added)

2. Select [] for the basic regular expression 2.1

[ABC] matches any character in the string ABC.

Available scope-

[A-Z]: match all lowercase English characters

[A-Z]: match all uppercase English characters

[0-9]: Match All numbers

[A-Za-Z]: match all English characters

2.2 .*

* In Bash has different meanings from regular expressions.

Regular Expression:

.: Represents any character

*: Indicates that there are 0 or more characters. For example, GX * can match one or more X characters After G: G, GX, gxx, gxxxx, etc,

End of the first line ^ $

^ Ex: Match ex to the beginning of a row

Tion $: Match tion at the end of a row

2.4 times {}

{N}	NIs a non-negative integer. MatchedNTimes.
{N,}	NIs a non-negative integer. At least matchNTimes.
{N,M}	MAndNAll are non-negative integers, whereN<=M. Least matchNTimes and most matchingMTimes.

Note that {} has special meaning in shell, so use \ escape as a regular expression.

3. Extended regular expressions are mainly used to add or | and () functions for basic regular expressions.

3.1 or |

Pattern1 | pattern2

Implement matching pattern1 or pattern2

We can also see that '|' has a higher priority than other characters (except parentheses of course)

Example: string matching string or char

grep -E 'string|char'

Matches string or char strings starting

grep -E '^string|^char'

3.2 () parentheses

It is the same as the parentheses you learned in elementary school.

Matches string or char strings starting

grep -E '^(string|char)'

4. Regular Expression

For more information about regular expressions, see:

Regular Expression

Grep and Regular Expression