Example usage of non-printing characters, special characters, and qualifiers (regular expression character set 2)

Non-printing characters

Character Meaning

\cx Matches the control character specified by x. For example, \cM matches a Control-M or carriage return character. The value of x must be one of A-Z or a-z. Otherwise, c is treated as a literal 'c' character.

\f Matches a form feed character. Equivalent to \x0c and \cL.

\n Matches a newline character. Equivalent to \x0a and \cJ.

\r Matches a carriage return character. Equivalent to \x0d and \cM.

\s Matches any whitespace characters, including spaces, tabs, form feeds, etc. Equivalent to [ \f\n\r\t\v].

\S Matches any non-whitespace characters. Equivalent to [^ \f\n\r\t\v].

\t Matches a tab character. Equivalent to \x09 and \cI.

\v Matches a vertical tab character. Equivalent to \x0b and \cK.

Special characters

The so-called special characters are characters with special meanings, such as the * in "*.txt" mentioned above. Simply put, it means The meaning of any string. If you want to find files with * in the file name, you need to escape the *, that is, add a \ before it. ls\*.txt. Regular expressions have the following special characters.

Special characters Description

$ Match the end of the input string. If the RegExp object's Multiline property is set, $ also matches '\n' or '\r'. To match the $ character itself, use \$.

( ) Marks the beginning and end of a subexpression. Subexpressions can be obtained for later use. To match these characters, use \( and \). ​

* ​ ​ ​ ​ Matches the preceding subexpression zero or more times. To match * characters, use \*. ​

+ ​ ​ ​ ​ Match the previous subexpression one or more times. To match the + character, use \+.

. Matches any single character except the newline character \n. To match ., use \. ​

[                   Marks the beginning of a square bracket expression. To match [, use \[. ​

? ​ ​ ​ ​ ​ Matches the preceding subexpression zero or one time, or specifies a non-greedy qualifier. To match the ? character, use \?. ​

\                   Mark the next character as either a special character, a literal character, a backward reference, or an octal escape character. For example, ‘n’ matches the character ‘n’. '\n' matches newline character. The sequence '\\' matches "\", while '\(' matches "(".

^                 Matches the beginning of the input string, unless used in a square bracket expression, in which case it means not Accepts this set of characters. To match the ^ character itself, use \^ . To match |, use \|. Regular expressions are constructed in the same way as mathematical expressions, using various metacharacters and operators. Expressions can be combined together to create larger expressions. The components of a regular expression can be a single character, a collection of characters, a range of characters, a selection between characters, or any combination of all of these components. Qualifier

The qualifier is used to specify how many times a given component of the regular expression must appear to satisfy the match. There are * or + or ? or {n} or {n,} or {. n, m} 6 types in total

The *, + and ? qualifiers are all greedy, because they will match as many words as possible. Non-greedy can be achieved by adding a ? after them. Or minimum match.

Regular expression qualifiers are:

Characters         Description      

                                                                                                                                        to Matches "z" and "zoo". * Equivalent to {0,}. , but cannot match "z". + is equivalent to {1,}

.

? Matches the preceding subexpression zero or one time. For example, "do(es)?" would match "do" in "do" or "does". ? Equivalent to {0,1}. ​

{n}                 n is a non-negative integer. Match a certain number of n times. For example, 'o{2}' cannot match the 'o' in "Bob", but it can match two o's in "food".

{n,} n is a non-negative integer. Match at least n times. For example, 'o{2,}' does not match the 'o' in "Bob", but it does match all o's in "foooood". ‘o{1,}’ is equivalent to ‘o+’. 'o{0,}' is equivalent to 'o*'.

{n,m} m and n are both non-negative integers, where n <= m. Match at least n times and at most m times. For example, "o{1,3}" will match the first three o's in "fooooood". ‘o{0,1}’ is equivalent to ‘o?’. Please note that there cannot be a space between the comma and the two numbers.

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