Shell Regular Expression

2. basic regular expression

   metacharacters	role
   *	matches the previous character 0 or more times
   .	match any character except newline characters
   ^	matches the beginning of the line. Reverse
   $	match line end
   []	matches any one of the characters set in brackets. For example: [0-9] matches any number. [A-z] matches any lowercase letter
   [^]	matches any character except the bracket character. For example: [^a-z] matches any one of the non-lowercase letters
   \	escape character. For example: grep "\.$" test.txt matches with a. End of the line
   \{n\}	means that the preceding characters appear exactly n times. For example, [0-9]\{4\} matches a 4-bit number. 1[3-8][0-9]\{9\} matches the phone number
   \{n,\}	indicates that the characters preceding it appear no less than n times. For example: [0-9]\{2,\} matches a number with at least 2 digits or more
   \{n,m\}	indicates that the characters preceding it appear at least n times and appear at most m times. For example: [a-z]\{4,6\} matches 4-bit to 6-bit lowercase letters

    

4. Example

   grep "A *" test.txt Note: It matches the "a" that appears 0 or more times, and all the content is output

   grep "aa*" test.txt matches a row that contains at least one a

   grep "s.. D "Text.txt matches lines with 2 characters between S and D

   grep "S.*d" Text.txt matches any character between S and D

   grep ". *" Text.txt matches all content

   Grep-n "^$" text.txt match empty line and lose travel number

   grep "^[a-z" matches lines that start with lowercase letters

   grep "^[^a-za-z" matches lines that do not start with a letter

Shell Regular Expression