Regex in a nutshell

regex

# egrep -i '^q[u]' testfile
# egrep -i '^q[^u]' testfile
quarter
qatre
A quarter mile.

# egrep -i '3[-./]19[-./]12' testfile 
3/19/12
3-19-12
3.19.12

# egrep -i '^(From|Subject|Date):' testfile --color
From: Bob
Subject: Hello
Date: today

# egrep -i 'ello\>' testfile 
shello
trello
fellow
mellow

# egrep -i 'July? (4(th)?|fourth)' testfile
July 4th
Jul 4th
Jul fourth
July 4 1966

# egrep -i '<hr( +size *= *"?.*"?)? */?>' testfile
<h1>
<h2>
</h2>

<h3    >
<h3 id="foo">
<hr>
<hr />
<hr size=12 >
<hr size="12px">


# Backreferences
# The following finds double words
# egrep -i '\<([a-z]+)\> +\1\>' testfile --color
This is the the way
To the theatre.

# egrep -i '\<([0-9]+)?(\.)?([0-9]+)?f?\>' testfile --color
10
10.0
10.12
10.
.12
.10f

$.10