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Reroute the blog link to the LLVM blog site. 

llvm-svn: 130014
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Greg Clayton 2011-04-22 21:03:40 +00:00
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<link href="style.css" rel="stylesheet" type="text/css" />
<title>LLDB FAQ</title>
</head>
<body>
<div class="www_title">
The <strong>LLDB</strong> Debugger
</div>
<div id="container">
<div id="content">
<!--#include virtual="sidebar.incl"-->
<div id="middle">
<div class="post">
<h1 class ="postheader"><div align="right" padding=20>April 20, 2011&nbsp;&nbsp;</div><div align="left">Regular Expression Commands</div></h1>
<div class="postcontent">
<p>Greetings LLDB users. I want to start writing regular blog posts
for the new and cool features and things you can do in LLDB. Today I
will start with one that was just added: <b>regular expression commands</b>.
What is a regular expression command? It is a command that you can
define by giving it a command name, where the command will have its raw arguments be
matched against one or more regular expressions. The first regular
expression to match your command's raw arguments wins and gets to
use substitutions to place argument snippets into a new command that
will get executed.</p>
<p>Lets supposed we want to use the <code>"f"</code> as a command to
select a frame by frame index. The command to select a frame by the
frame index is the <code>"frame select &lt;num&gt;"</code> command
which you might not always want to type out (even with out command
completion).
We can make this a bit easier this using a regular expression command:
<p>
<code>(lldb) command regex f 's/([0-9]+)/frame select %1/'</code>
</p>
<p><code>"command regex f"</code> tells the interpreter to create a new
regex command named <code>"f"</code>, and when a command is entered
on the command line that starts with <code>"f"</code>, it will match
the remaining command text against the regular expression
<code>"([0-9]+)"</code>, and it if matches, it will substitute any
parenthesized subexpressions as needed. Here we enclosed the number
regular expression <code>"[0-9]+"</code> in parentheses which will
save the results in the first match and allow the matching string
to be substitued into the result <code>"frame select %1"</code>.
So now we can use our new command and it will show us what command
resulted from our regular expression substitution:
<p>
<p>
<code>(lldb) f 12<br>
frame select 12
</code>
</p>
<p>Leading spaces will always be stripped, but there may be trailing spaces
since we are processing the remaining raw command string that follows
the initial <code>f</code> command name, plus any leading spaces. The regular expression
is also just looking for any sequence of one or more digits. Our
current regular expression will actually match <code>"f 11 22 33"</code>:
<p>
<code>(lldb) f 11 22 33<br>
frame select 11
</code>
</p>
Since this isn't desired, we should make the regular expression more complete by checking for
the start of the line (<code>^</code>) and the end of the line (<code>$</code>)
and also allow for zero or more spaces (<code>[[:space:]]*</code>)
to come after the number. Our newer and safer regular expression command line looks like:
<p>
<code>(lldb) command regex f 's/^([0-9]+)[[:space:]]*$/frame select %1/'</code>
</p>
<p>Now we can type in a command as <code>"f 12 "</code> (note the trailing
spaces), and still get correct substitutions, while our previous
example of <code>"f 11 22 33"</code> will no longer match:
<p>
<code>(lldb) f 11 22 33<br>
error: Command contents '11 22 33' failed to match any regular expression in the 'f' regex command.
</code>
<p>Lets take this a bit further by also using the <code>f</code> command
to emulate GDB's <code>finish</code> command when it is typed without
any arguments. We will also modify this command to watch for a single
"+" or "-" followed by a digit to signify a relative frame change
using the frame select command with the --relative option:
</p>
<p>
<code>(lldb) frame select --relative &lt;offset&gt;</code>
<p>
Multiple regular expressions can be entered in on the command line
or using the multi-line mode when typing in a live LLDB debug session.
Below the text in bold is user entered:
<p>
<code>
(lldb) <b>commands regex f</b><br>
Enter regular expressions in the form 's/&lt;regex&gt;/&lt;subst&gt;/' and terminate with an empty line:<br>
<b>s/^([0-9]+)[[:space:]]*$/frame select %1/<br>
s/^([+-][0-9]+)[[:space:]]*$/frame select --relative=%1/<br>
s/^[[:space:]]*$/finish/</b><br>
<br>
(lldb) <b>f</b><br>
finish<br>
...<br>
(lldb) <b>f -1</b><br>
frame select --relative=-1<br>
...<br>
(lldb) <b>f +1</b><br>
frame select --relative=+1<br>
...<br>
(lldb) <b>f 12</b><br>
frame select 12<br>
</code>
</p>
<p>I hope you can see the possilbities in how you can customize your
command line experience in LLDB using these commands. You can add
any regular expression commands to your <b>~/.lldbinit</b> file to
always have your regular expression commands defined in your debug
sessions.
</div>
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<h1 class="headerbar">Goals and Status</h1>
<ul>
<li><a href="index.html">About</a></li>
<li><a href="blog.html">Blog</a></li>
<li><a href="http://blog.llvm.org/search/label/LLDB">Blog</a></li>
<li><a href="goals.html">Goals</a></li>
<li><a href="features.html">Features</a></li>
<li><a href="status.html">Status</a></li>