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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
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      retrieved online at http://www.erlang.org/.
    
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    <title>Pattern Matching</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>patterns.xml</file>
  </header>

  <section>
    <title>Pattern Matching</title>
    <p>Variables are bound to values through the <em>pattern matching</em> mechanism. Pattern matching occurs when
      evaluating a function call, <c>case</c>- <c>receive</c>-
      <c>try</c>- expressions and match operator (=) expressions.</p>
    <p>In a pattern matching, a left-hand side
      <seealso marker="expressions#pattern">pattern</seealso> is matched
      against a right-hand side
      <seealso marker="expressions#term">term</seealso>. If
      the matching succeeds, any unbound variables in the pattern
      become bound. If the matching fails, a run-time error occurs.</p>
    <p>Examples:</p>
    <pre>
1> <input>X.</input>
** 1: variable 'X' is unbound **
2> <input>X = 2.</input>
2
3> <input>X + 1.</input>
3
4> <input>{X, Y} = {1, 2}.</input>
** exception error: no match of right hand side value {1,2}
5> <input>{X, Y} = {2, 3}.</input>
{2,3}
6> <input>Y.</input>
3</pre>
  </section>
</chapter>

