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Regular expressions are divided into two categories: basic regular expressions and extended regular expressions.

Basic regular expression metacharacters:

Character matching:

.: matches any character.

[]: matches any character in the specified range in [].

[^]: any character outside the specified range; you can also know that in [] parentheses, ^ indicates inversion.

Number of times match:

*: the character before it appears any number of times.

\?: the preceding character appears 0 or 1 times

\ {mdirection n}: the first character appears at least m times and up to n times. If there is no upper limit, n can not be written, and if it occurs at least 0 times, m is 0. M cannot be omitted.

. *: any character of any length appears before it.

Character Mou Ding:

^: the head of the line is Mou Ding.

&: Mou Ding at the end of the line.

\

Or\ b: the word Mouding is at the end of the line

\ (\): used for grouping. Backward reference:\ 1,\ 2,\ 3. Indicates the contents in parentheses of the nth pair of references

Grep: commands that use patterns defined by basic regular expressions to filter text are supported.

-I: ignore the case of matching letters

-v: reverse matching, displayed as matching content

-o: only the matching content is displayed

-- color: color display

-E=eprep: use extended regularities for matching.

-A digits,-B digits,-C digits:-A (after) shows not only the matching lines, but also the next N rows that match. -B (before) indicates that the first N lines of the match are displayed at the same time, and-C indicates that the first N lines and the last N lines are displayed at the same time.

More options are viewed in man.

To be clear, basic regular expressions work in greedy mode.

Extended regular expression

Character matching, position determined: same as basic regular

.

[]

[^]

Number of times match:

*: the same as the basic regular, the characters preceding it appear any number of times.

? Same as the basic regular, the preceding character appears 0 or 1 times

+: an extra + matches its first character at least once. Equivalent to\ {1,\}? + = *

{mdirection n}: you don't need to use\ to change your mind in the extension rule.

Grouping and

(): grouping

1, 2, 3. Backward reference

Or

|: | the front and rear are each as a whole. For example, abc | dec: indicates abc or dec. You can use egrep-- color for verification. To indicate c or d but want (), ab (c | d) ec

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