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<div class="example" id="regular-expressions">
<h2><a href="./">Go by Example</a>: Regular Expressions</h2>
<a href="http://play.golang.org/p/7bnr2EXlxK"><img title="Run code" src="play.png" class="run" /></a>
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<p>Go offers built-in support for <a href="http://en.wikipedia.org/wiki/Regular_expression">regular expressions</a>.
Here are some examples of common regexp-related tasks
in Go.</p>
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"bytes"</span>
<span class="kn">import</span> <span class="s">"fmt"</span>
<span class="kn">import</span> <span class="s">"regexp"</span>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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<p>This tests whether a pattern matches a string.</p>
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<div class="highlight"><pre> <span class="nx">match</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="nx">regexp</span><span class="p">.</span><span class="nx">MatchString</span><span class="p">(</span><span class="s">"p([a-z]+)ch"</span><span class="p">,</span> <span class="s">"peach"</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">match</span><span class="p">)</span>
</pre></div>
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<p>Above we used a string pattern directly, but for
other regexp tasks you’ll need to <code>Compile</code> an
optimized <code>Regexp</code> struct.</p>
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<div class="highlight"><pre> <span class="nx">r</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="nx">regexp</span><span class="p">.</span><span class="nx">Compile</span><span class="p">(</span><span class="s">"p([a-z]+)ch"</span><span class="p">)</span>
</pre></div>
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<p>Many methods are available on these structs. Here’s
a match test like we saw earlier.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">MatchString</span><span class="p">(</span><span class="s">"peach"</span><span class="p">))</span>
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<p>This finds the match for the regexp.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindString</span><span class="p">(</span><span class="s">"peach punch"</span><span class="p">))</span>
</pre></div>
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<p>The also finds the first match but returns the
start and end indexes for the match instead of the
matching text.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindStringIndex</span><span class="p">(</span><span class="s">"peach punch"</span><span class="p">))</span>
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<p>The <code>Submatch</code> variants include information about
both the whole-pattern matches and the submatches
within those matches. For example this will return
information for both <code>p([a-z]+)ch</code> and <code>([a-z]+)</code>.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindStringSubmatch</span><span class="p">(</span><span class="s">"peach punch"</span><span class="p">))</span>
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<p>Similarly this will return information about the
indexes of matches and submatches.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindStringSubmatchIndex</span><span class="p">(</span><span class="s">"peach punch"</span><span class="p">))</span>
</pre></div>
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<p>The <code>All</code> variants of these functions apply to all
matches in the input, not just the first. For
example to find all matches for a regexp.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindAllString</span><span class="p">(</span><span class="s">"peach punch pinch"</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">))</span>
</pre></div>
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<p>These <code>All</code> variants are available for the other
functions we saw above as well.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindAllStringSubmatchIndex</span><span class="p">(</span>
<span class="s">"peach punch pinch"</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">))</span>
</pre></div>
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<p>Providing a non-negative integer as the second
argument to these functions will limit the number
of matches.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">FindAllString</span><span class="p">(</span><span class="s">"peach punch pinch"</span><span class="p">,</span> <span class="mi">2</span><span class="p">))</span>
</pre></div>
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<p>Our examples above had string arguments and used
names like <code>MatchString</code>. We can also provide
<code>[]byte</code> arguments and drop <code>String</code> from the
function name.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">Match</span><span class="p">([]</span><span class="nb">byte</span><span class="p">(</span><span class="s">"peach"</span><span class="p">)))</span>
</pre></div>
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<p>When creating constants with regular expressions
you can use the <code>MustCompile</code> variation of
<code>Compile</code>. A plain <code>Compile</code> won’t work for
constants because it has 2 return values.</p>
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<div class="highlight"><pre> <span class="nx">r</span> <span class="p">=</span> <span class="nx">regexp</span><span class="p">.</span><span class="nx">MustCompile</span><span class="p">(</span><span class="s">"p([a-z]+)ch"</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">)</span>
</pre></div>
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<p>The <code>regexp</code> package can also be used to replace
subsets of strings with other values.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">r</span><span class="p">.</span><span class="nx">ReplaceAllString</span><span class="p">(</span><span class="s">"a peach"</span><span class="p">,</span> <span class="s">"<fruit>"</span><span class="p">))</span>
</pre></div>
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<p>The <code>Func</code> variant allows you to transform matched
text with a given function.</p>
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<div class="highlight"><pre> <span class="nx">in</span> <span class="o">:=</span> <span class="p">[]</span><span class="nb">byte</span><span class="p">(</span><span class="s">"a peach"</span><span class="p">)</span>
<span class="nx">out</span> <span class="o">:=</span> <span class="nx">r</span><span class="p">.</span><span class="nx">ReplaceAllFunc</span><span class="p">(</span><span class="nx">in</span><span class="p">,</span> <span class="nx">bytes</span><span class="p">.</span><span class="nx">ToUpper</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nb">string</span><span class="p">(</span><span class="nx">out</span><span class="p">))</span>
<span class="p">}</span>
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<div class="highlight"><pre><span class="gp">$</span> go run regular-expressions.go
<span class="go">true</span>
<span class="go">true</span>
<span class="go">peach</span>
<span class="go">[0 5]</span>
<span class="go">[peach ea]</span>
<span class="go">[0 5 1 3]</span>
<span class="go">[peach punch pinch]</span>
<span class="go">[[0 5 1 3] [6 11 7 9] [12 17 13 15]]</span>
<span class="go">[peach punch]</span>
<span class="go">true</span>
<span class="go">p([a-z]+)ch</span>
<span class="go">a <fruit></span>
<span class="go">a PEACH</span>
</pre></div>
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<p>For a complete reference on Go regular expressions check
the <a href="http://golang.org/pkg/regexp/"><code>regexp</code></a> package docs.</p>
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