Regular expression notes

one。 Brief introduction:

A regular expression is basically a "representation", and as long as the tool program supports this representation, it can be used as a string processing of a regular expression. Tools such as vi,grep,awk,sed because they support regular expressions. A regular expression is an arrangement of special characters used to find, replace, and delete some or more lines of text strings. To put it simply, a regular expression is an "expression" used in string processing. It's really convenient to use it. So study hard.

two。 Basic type:

1. Character matching metacharacter:

. (period): matches any character

[]: matches one of the characters in parentheses

[^]: match other characters that do not include characters in parentheses

two。 Times match metacharacters:

*: match the previous characters any number of times (including zero)

\?: match the previous character zero or once (the preceding character is optional)

\ +: match the preceding characters at least once

\ {n\}: matches the preceding character n times

\ {n,\}: matches the preceding character at least n times

\ {, m\}: matches the preceding character at most m times (including zero times)

\ {NMagol m\}: matches the preceding character at least n times up to m times.

3. Position description metacharacter:

^: describe the beginning of the line

$: draw the end of the line

\

< (\b)：描定词首（出现在表达式首部）； \>

(\ b): describe the suffix (appears at the end of the expression)

Grouping: used to store matching characters in memory variables for later reference.

\ (\): grouping symbol

\ 1: reference the character matched by the first group

4. Schedule:

Character

Description

\

Marks the next character as a special character, or a literal character, or a backward reference, or an octal escape character. For example,'n 'matches the character "n".' \ n' matches a newline character. The sequence'\ 'matches "\" and "(" matches "(".

^

Matches the starting position of the input string. If the Multiline property of the RegExp object is set, ^ also matches the position after'\ n'or'\ r'.

$

Matches the end position of the input string. If the Multiline property of the RegExp object is set, $also matches the position before'\ n'or'\ r'.

*

Matches the previous subexpression zero or more times. For example, zo* can match "z" and "zoo". * is equivalent to {0,}.

+

Matches the previous subexpression one or more times. For example, 'zo+' can match "zo" and "zoo", but not "z". + is equivalent to {1,}.

?

Matches the previous subexpression zero or once. For example, "do (es)?" Can match "do" in "do" or "does". ? It is equivalent to {0jue 1}.

{n}

N is a non-negative integer. Match the determined n times. For example,'o {2} 'does not match the' o'in 'Bob', but does match the two o in 'food'.

{n,}

N is a non-negative integer. Match at least n times. For example,'o {2,} 'does not match' o'in 'Bob', but does match all o in 'foooood'. O {1,}'is equivalent to 'oasis'. O {0,}'is equivalent to 'oval'.

{n,m}

M and n are non-negative integers, where n