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<h1 id="title-xs"><a href="index.html">The Common Lisp Cookbook</a> – Regular Expressions</h1>
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<div id="toc-title">Table of Contents</div>
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<h1 id="title-non-xs"><a href="index.html">The Common Lisp Cookbook</a> – Regular Expressions</h1>
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📕 <a href="index.html#download-in-epub">Get the EPUB and PDF</a>
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<div id="content"
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<p>The <a href="http://www.lispworks.com/documentation/HyperSpec/index.html">ANSI Common Lisp
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standard</a>
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does not include facilities for regular expressions, but a couple of
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libraries exist for this task, for instance:
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<a href="https://github.com/edicl/cl-ppcre">cl-ppcre</a>.</p>
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<p>See also the respective <a href="http://www.cliki.net/Regular%20Expression">Cliki:
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regexp</a> page for more
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links.</p>
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<p>Note that some CL implementations include regexp facilities, notably
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<a href="http://clisp.sourceforge.net/impnotes.html#regexp">CLISP</a> and
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<a href="https://franz.com/support/documentation/current/doc/regexp.htm">ALLEGRO
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CL</a>. If
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in doubt, check your manual or ask your vendor.</p>
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<p>The description provided below is far from complete, so don’t forget
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to check the reference manual that comes along with the CL-PPCRE
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library.</p>
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<h2 id="ppcre">PPCRE</h2>
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<h3 id="using-ppcre">Using PPCRE</h3>
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<p><a href="https://github.com/edicl/cl-ppcre">CL-PPCRE</a> (abbreviation for
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Portable Perl-compatible regular expressions) is a portable regular
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expression library for Common Lisp with a broad set of features and
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good performance. It has been ported to a number of Common Lisp
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implementations and can be easily installed (or added as a dependency)
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via Quicklisp:</p>
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<pre><code class="language-lisp">(ql:quickload "cl-ppcre")
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</code></pre>
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<p>Basic operations with the CL-PPCRE library functions are described
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below.</p>
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<h3 id="looking-for-matching-patterns">Looking for matching patterns</h3>
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<p>The <code>scan</code> function tries to match the given pattern and on success
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returns four multiple-values values - the start of the match, the end
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of the match, and two arrays denoting the beginnings and ends of
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register matches. On failure returns <code>NIL</code>.</p>
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<p>A regular expression pattern can be compiled with the <code>create-scanner</code>
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function call. A “scanner” will be created that can be used by other
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functions.</p>
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<p>For example:</p>
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<pre><code class="language-lisp">(let ((ptrn (ppcre:create-scanner "(a)*b")))
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(ppcre:scan ptrn "xaaabd"))
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</code></pre>
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<p>will yield the same results as:</p>
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<pre><code class="language-lisp">(ppcre:scan "(a)*b" "xaaabd")
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</code></pre>
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<p>but will require less time for repeated <code>scan</code> calls as parsing the
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expression and compiling it is done only once.</p>
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<h3 id="replacing-text">Replacing text</h3>
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<pre><code class="language-lisp">(ppcre:regex-replace "a" "abc" "A") ;; => "Abc"
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;; or
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(let ((pat (ppcre:create-scanner "a")))
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(ppcre:regex-replace pat "abc" "A"))
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</code></pre>
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<h3 id="extracting-information">Extracting information</h3>
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<p>CL-PPCRE provides a several ways to extract matching fragments, among
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them: the <code>scan-to-strings</code> and <code>register-groups-bind</code> functions.</p>
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<p>The <code>scan-to-strings</code> function is similar to <code>scan</code> but returns
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substrings of target-string instead of positions. This function
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returns two values on success: the whole match as a string plus an
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array of substrings (or NILs) corresponding to the matched registers.</p>
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<p>The <code>register-groups-bind</code> function tries to match the given pattern
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against the target string and binds matching fragments with the given
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variables.</p>
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<pre><code class="language-lisp">(ppcre:register-groups-bind (first second third fourth)
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("((a)|(b)|(c))+" "abababc" :sharedp t)
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(list first second third fourth))
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;; => ("c" "a" "b" "c")
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</code></pre>
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<p>CL-PPCRE also provides a shortcut for calling a function before
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assigning the matching fragment to the variable:</p>
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<pre><code class="language-lisp">(ppcre:register-groups-bind (fname lname (#'parse-integer date month year))
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("(\\w+)\\s+(\\w+)\\s+(\\d{1,2})\\.(\\d{1,2})\\.(\\d{4})" "Frank Zappa 21.12.1940")
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(list fname lname (encode-universal-time 0 0 0 date month year 0)))
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;; => ("Frank" "Zappa" 1292889600)
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</code></pre>
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<h3 id="syntactic-sugar">Syntactic sugar</h3>
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<p>It might be more convenient to use CL-PPCRE with the
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<a href="https://github.com/edicl/cl-interpol">CL-INTERPOL</a>
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library. CL-INTERPOL is a library for Common Lisp which modifies the
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reader in a way that introduces interpolation within strings similar
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to Perl, Scala, or Unix Shell scripts.</p>
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<p>In addition to loading the CL-INTERPOL library, initialization call
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must be made to properly configure the Lisp reader. This is
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accomplished by either calling the <code>enable-interpol-syntax</code> function
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from the REPL or placing that call in the source file before using any
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of its features:</p>
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<pre><code class="language-lisp">(interpol:enable-interpol-syntax)
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</code></pre>
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<p class="page-source">
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Page source: <a href="https://github.com/LispCookbook/cl-cookbook/blob/master/regexp.md">regexp.md</a>
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