基礎正規表示法

重要特殊字元(characters)

RE字符         意義與範例
^word    待搜尋的字串(word)在行首。
word$    待搜尋的字串(word)在行尾。
  .      代表『任意一個』字符，一定是一個任意字符。
  \      跳脫字符，將特殊符號的特殊意義去除。
  *      重複零個或多個的前一個 RE 字符
\{n,m\}  連續 n 到 m 個的『前一個 RE 字符』
         若為 \{n\} 則是連續 n 個的前一個 RE 字符，
         若是 \{n,\} 則是連續 n 個以上的前一個 RE 字符。
  [ ]    在 [ ] 當中『僅代表一個待搜尋的字元』
         若 [^]，^ 在中括號中表示 NOT。

找檔案 /etc/man.config 中行首是大寫 C 的行。

[dywang@dywmac ~]$ grep -n ^C /etc/man.config 
108:CAT		/bin/cat
114:CMP		/usr/libexec/man-cmp.sh
118:COMPRESS	/usr/bin/lzma
119:COMPRESS_EXT	.lzma

查詢行尾是 s.。

[dywang@dywmac ~]$ grep -n 's\.$' /etc/man.config
37:# and to determine the correspondence between extensions and decompressors.
63:# lots of other nearby files and directories.
144:# Enable/disable makewhatis database cron updates.

計算行首是 # 且只有這個註解符號，共有 34 行。
```
[dywang@dywmac ~]$ grep '^#$' /etc/man.config | wc -l
34
```
計算行首不是 # 的行數，也就是不是註解的行，共有 39 行。
```
[dywang@dywmac ~]$ grep -v '^#' /etc/man.config | wc -l
113
```

搜尋 C 接 H 或 M

[dywang@dywmac ~]$ grep -n 'C[HM]' /etc/man.config
80:# NOCACHE keeps man from creating cache pages ("cat pages")
84:#NOCACHE
111:# When CMP is defined man will try to avoid showing the same
114:CMP		/usr/libexec/man-cmp.sh

搜尋行首 C 但不接 H 或 M

[dywang@dywmac ~]$ grep -n '^C[^HM]' /etc/man.config
108:CAT		/bin/cat
118:COMPRESS	/usr/bin/lzma
119:COMPRESS_EXT	.lzma

搜尋數字後至少接一個以上英文字母

[dywang@dywmac ~]$ grep '[0-9][a-zA-Z][a-zA-Z]*' /etc/man.config
# man.conf from man-1.6f
MANPATH	/usr/X11R6/man
MANPATH_MAP	/usr/X11R6/bin		/usr/X11R6/man
MANPATH_MAP	/usr/bin/X11		/usr/X11R6/man
# and the MANSECT environment variable is not set (1x-8x sections are used by
MANSECT		1:1p:8:2:3:3p:4:5:6:7:9:0p:n:l:p:o:1x:2x:3x:4x:5x:6x:7x:8x

搜尋連續 12 到 15 個英文字母的字串

[dywang@dywmac ~]$ grep '[a-zA-Z]\{12,15\}' /etc/man.config
# Generated automatically from man.conf.in by the
# pages corresponding to given man pages should be stored,
# MANPATH		manpath_element	[corresponding_catdir]
# and to determine the correspondence between extensions and decompressors.
# Every automatically generated MANPATH includes these fields
# NOAUTOPATH keeps man from automatically adding directories that look like
#MANDEFOPTIONS	-a
# Decompress with given decompressor when input file has given extension
# If MAKEWHATISDBUPDATES variable is uncommented
#MAKEWHATISDBUPDATES	n

搜尋連續 14 個以上的英文字母

[dywang@dywmac ~]$ grep '[a-zA-Z]\{14,\}' /etc/man.config
# and to determine the correspondence between extensions and decompressors.
# If MAKEWHATISDBUPDATES variable is uncommented
#MAKEWHATISDBUPDATES	n

搜尋剛好連續 13 個英文字母

[dywang@dywmac ~]$ grep '[^a-zA-Z][a-zA-Z]\{13\}[^a-zA-Z]' /etc/man.config
# Generated automatically from man.conf.in by the
# pages corresponding to given man pages should be stored,
# MANPATH		manpath_element	[corresponding_catdir]
# and to determine the correspondence between extensions and decompressors.
# Every automatically generated MANPATH includes these fields
# NOAUTOPATH keeps man from automatically adding directories that look like
#MANDEFOPTIONS	-a

搜尋 x 開始 y 結束的字串

[dywang@dywmac ~]$ grep 'x.*y' /etc/man.config
# Explicitly given catdirs override.
# The command "man -a xyzzy" will show all man pages for xyzzy.
# and the MANSECT environment variable is not set (1x-8x sections are used by

搜尋多個字串，例如：this 及 will 兩個字串。

[root@dywssd bin]# grep 'this\|will' /etc/man.config 
# For more information about this file, see the man pages man(1)
# (so that this dir has both man1 etc. and cat1 etc. subdirs).
# The keyword FSSTND will cause this behaviour.
# The keyword FHS will cause this behaviour (and overrides FSSTND).
# in the mandatory manpath already, but will keep man from statting
# The command "man -a xyzzy" will show all man pages for xyzzy.
# When CMP is defined man will try to avoid showing the same
# will not update makewhatis database.
# Otherwise the database will be updated.