Regular Expression note

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Match dictionary letters
The dot (.) matches any character other than line breaks and null, for example, 'a. B 'matches a string of three characters starting with a and ending with B.
After B is followed by a, the result is a regular expression that matches a string. If a matches the second part, B matches the second part.

Repeated operators '*', '+ ','? '
* Matches zero or zero times. 'fo' matches 'F', 'fo', and 'foo '.
+ Match one or more times
? Matches zero times or once

Interval operator {} Or /{/}
{Count} matches the exact count times with the words in the previous regular expression.
{Min,} matches words that match the Regular Expression in minutes or more times.
{Min, max} matches at least min times, but no more than Max times meet the previous regular expression word
The following situations are invalid:
1. Min is greater than Max
2. Any count, Max, and Min values outside 0 and're _ dup_max'

Select operator |
Any condition in the expression will be matched to 'foo | bar | quux ', such as 'foo', 'bar', or 'quux'

List operator []
Match one of the characters listed in brackets, such as [AB], to match A or B
The mismatch list is given by "[^ character]", for example, [^ AB] matches all characters except a and B.
The characters in brackets are called lists. special characters in the list have different meanings. If ']' appears, escape
'[:' Indicates the start character class operator. If're _ char_classes 'is set, a valid character class expression is followed.
':]' Indicates the operator of the closed character class. If're _ char_classes 'is set, the start character operator before it is formed by the operator of the valid character class plus the end character operator.
'-' Indicates the range operator.
All other characters are dictionary characters (not escaped) such as '[. *]' matching. And '*'
Note: '^' is not the starting character in the list.

Character class operator [:]
If the semantic bit're _ character_classes 'is set, the Regular Expression Engine can recognize the character class expressions in the list.
Format: [: Class-Name:]
The character class name and its meaning are as follows:
Alnum letters and numbers
Alpha letter
Blank system related. GNU system, a space or a tab
Cntrl control character, ASCII code, code value 0177 and code value less than 040
Digit numbers
The graph and print are the same except for spaces.
Lower-case lower letters
Print printable characters, ASCII encoding, space, waveform symbol-code value: 040 to 0176
Punct is neither a control nor a letter character
Space press enter, line feed, vertical tab, form feed
Upper case letters
Xdigit hexadecimal number

Range operator '-'
The range expression in the Regular Expression Engine recognition list (list), for example, [A-F], indicates the character ranges including A and F.
If re_no_empty_ranges is set, it is invalid if the sequence of the start point table is greater than the end point, for example, [Z-A]. If this bit is not set, the expression considers this interval to be null.
A. if you want to use '-' in the list, place it at the beginning or end of the list.
B. contains a range so that the actual vertex comparison table is smaller than '-', and its ending point is equal to or greater than '-', unless a range is the first element in the list, otherwise, '-' cannot be its starting point, but can be its ending point. This is because the regular engine treats '-' as an interval operator unless it has another '-' (include a range whose starting point collates strictly lower than '-' and whose ending point collates equal or higher. unless a range is the first item in a list, a'-'Can't be its starting point, but _ Can _ be its ending point. that is because RegEx considers '-' to be the range operator unless it is preceded by another '-'.)
For example, in the ASCII code, ')', '*', '+ ',',','-','. the ','/'character is a continuous character (in the ABCD format) in the sequence table, for example,' [)-+ --/] 'interval ') -+ 'and' + -- 'plus the character'/', not')-+'
And '--/'.
C. Place the range operator '-' in the first element of the list,
For example, '[-a-Z]' matches lowercase letters or a font size.

Grouping operator ()
Grouping operators are called subexpressions. Regular Expressions regard the sequence in parentheses together with parentheses as a unit.
You can use groups:
A. delimit the argument (s) to an alternation operator or a repetition operator
B. Keep track of the indices of the substring that matched a given
Group.

Post-reference operator (the escape character we often call )/
N

Positioning Operator
'^' Matches the first line operator
'$' Matches the end operator.

GNU Operator
'/B' word boundary matching Operator
'/B' intra-word matching Operator
'/<' Start matching Operator
'/>' End matching Operator
'/W' match-word-constituent operato
'/W' match-non-word-constituent OPERATOR: