grep.markdown

GNU grep Notes

grep = g/RE/p

BRE vs. ERE

In GNU grep, there is no difference in available functionality between basic and extended syntaxes; in other impletions, basic regular expressions are less powerful.

In basic regular expressions the meta-characters ? + | { ( and ) lose their special meaning; instead use the backslashed verson: \? \+ \| \{ \( \).

via man 1 grep

Some Tests

$ grep -p '[[.ch.]]' Regex.txt
grep:POSIX collating elements are not supported.

$ grep -E "3{2}" Regex.txt		# OK
$ grep "3{2}" Regex.txt			# Error	
$ grep "3\{2\}" Regex.txt		# OK

$ grep "[a-z]" Regex.txt (-o)	#每个match都为单个字母。
$ grep "[a-z]*" Regex.txt (-o)	#每个match多为连续的字母序列。

$ grep '[^0-9]' datafile		#不是打印“不含数字的行”，而是打印
								#所有“包含一个非数字存在的行”。

$ grep 'ss* ' datafile			#代表s后跟0或多个连续的s, 再跟一空格;
								#`s*`是一个整体，不是`s+*`

$ grep '\<[a-z].*n\>' datafile		# \< \>成对出现并不限定一个单词
northwest	NW	Charles Main	

$ grep -w '\[a-z].*n\n' datafile	#结果同上(The match forms whole words!)
									#As defined in `man 1 grep`

$ grep -i "$LOGNAME" datafile		#变量会被Shell展开;
$ grep -i '$LOGNAME' datafile		#变量不会被Shell展开

$ grep '^n\w*\W' datafile			#GNU grep支持\w, \W
$ grep '\bnorth\b'	datafile		#和\b