Regular Expressions- RE簡介(選項)

|代表"或" :
 底下三個表達式，全部代表相同意思
Jeffrey|Jeffery
Jeff(rey|ery)
Jeff(re|er)y

 底下四個表達式，全部代表相同意思
(Geoff|Jeff)(rey|ery)
(Geo|Je)ff(rey|ery)
(Geo|Je)ff(re|er)y
Jeffrey|Geoffery|Jeffery|Geoffrey

^From|Subject|Date:. 與^(From|Subject|Date):.有非常大的差異
 ^(From|Subject|Date):. 他會匹配:
1. 行首From:.
2. 行首Subject:.
3. 行首Date:.


 [root@www mail]# egrep '^(From|Subject|Date): ' root
Date: Thu, 23 Aug 2012 10:38:12 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Fri, 24 Aug 2012 10:21:47 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Mon, 27 Aug 2012 10:11:18 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Tue, 28 Aug 2012 10:20:02 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Wed, 29 Aug 2012 14:15:20 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Thu, 30 Aug 2012 16:50:30 +0800
From: logwatch@localhost.localdomain
Subject: Logwatch for www.eric.com.tw (Linux)
Date: Fri, 31 Aug 2012 14:27:30 +0800
From: logwatch@localhost.localdomain