正则表达式

Basic Patterns

The power of regular expressions is that they can specify patterns,not just fixed characters. Here are the most basic patterns which match single chars:

a,X,9,< -- ordinary characters just match themselves exactly. The meta-characters which do not match themselves because they have special meanings are: . ^ $ * + ? { [ ] | ( ) (details below)

. (a period) -- matches any single character except newline 'n'

w -- (lowercase w) matches a "word" character: a letter or digit or underbar [a-zA-Z0-9_]. Note that although "word" is the mnemonic for this,it only matches a single word char,not a whole word. W (upper case W) matches any non-word character.

b -- boundary between word and non-word

s -- (lowercase s) matches a single whitespace character -- space,newline,return,tab,form [ nrtf]. S (upper case S) matches any non-whitespace character.

t,n,r -- tab,return

d -- decimal digit [0-9] (some older regex utilities do not support but d,but they all support w and s)

^ = start,$ = end -- match the start or end of the string

-- inhibit the "specialness" of a character. So,for example,use . to match a period or to match a slash. If you are unsure if a character has special meaning,such as '@',you can put a slash in front of it,@,to make sure it is treated just as a character.

Repetition

Things get more interesting when you use + and * to specify repetition in the pattern

• + -- 1 or more occurrences of the pattern to its left,e.g. 'i+' = one or more i's
• * -- 0 or more occurrences of the pattern to its left
• ? -- match 0 or 1 occurrences of the pattern to its left