14. Study progress and task of self-study--linux "Regular expression"

Regular expressions

grep, Egrep, and fgrep of text processing tools

1, the basic regular expression

　　　　　　grep: (Global search Regular expression (RE) and print out of the line)
Text Search tool to search the target file on a row-by-line basis based on user-specified text patterns and display matching rows

　　　　　　　　# grep [options] ' Patterf (mode) ' file, default only supports basic regular expressions, characters to match extra features with-E
--color=auto Specifying Colors
-V: Reverse match, showing rows that cannot be matched by the pattern
-O: Matches only strings that are matched to the pattern, not the entire row
-I: Case insensitive, ignore-case
-E: Support extended
-A #: also displays the following line in the mode
-B #: Also displays a row above the pattern
-C #: Display one row before and after each

Regular expression: is the pattern (pattern) written by a class of characters
Metacharacters: Similar to wildcard characters, does not represent the meaning of the character itself, for the description of additional functionality

The metacharacters of the basic regular expression:

character match:
.: Any single character
[]: any single character within the specified range 　　　　　　　　　　　　　　　　
[0-9],[[:d Igit:]]
[A-z],[[:lower:]]
[A-z],[[:upper:]]
all letters: [[: Alpha:]]
letter + Number: [[: alnum:]]
space: [[: Space:]]
Punctuation: [[:p UTCT:]]
[^]: Specify any single character outside the range

Number of occurrences: used to specify the number of occurrences of the preceding character
*: Any time
Example: X*y xxy xy y can match
. *: Matches any character of any length
\?: The preceding character appears 0 or 1 times
\{m\}: Matches the preceding character m times
\{m,n\}: At least m times, up to N times
\{m,\}: At least m times
\{0,n}: Up to n times
Use "" To do variable conversions

Greedy mode: to match characters as long as possible:

Position anchoring: Used to specify where characters appear
^: Anchoring the beginning of the line
^char
$: Anchor Line End
grepchar$
^$: Blank Line

The position of the word is anchored:

\<char: Anchor word head, \bchar
Char\>: Anchor ending, char\b

Group:
\(\)
\ (ab\) *xy

Reference:
\1: A back reference, referring to the preceding first opening parenthesis and the corresponding pattern in the right parenthesis to match the content, meaning that the previous occurrence, in \1 this also appears once
\2
...
\ (A.B) xy\1:a6bxya6b

2. Extended Regular Expressions

　　　　Egrep: Use an extended regular expression to build the pattern, which is equivalent to GREP-E
Metacharacters
Character Matching:
.: Any single character
[]: Any single character within the specified range
[^]: Any single character outside the specified range

Number of matches:
*: The character in front of the match is any time
？ : The character in front of the match 0 or 1 times
+: Match its preceding character at least once
{m}: matches its preceding character m times
{M,n}: At least m times, up to N times
{m,}: at least m times
{0,n}: up to n times

Position anchoring: Used to specify where characters appear
^: Anchoring the beginning of the line
^char
$: Anchor Line End
grepchar$
^$: Blank Line

The position of the word is anchored:
\<char: Anchor word head, \bchar
Char\>: Anchor ending, char\b

Group:
(): Group
|: Or, AC|BC ac or BC

　　Fgrep: Non-parsing of regular expressions

14. Study progress and task of self-study--linux "Regular expression"