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copied!<p>Thanks for the answers. I feel each answer had part of the answer. Here is what I was looking for.</p> <ol> <li><p><strong>?</strong> symbol is just a shorthand for <strong>(something|ε)</strong>. Thus <strong>(a|ε)</strong> can be rewritten as <strong>a?</strong>. So the example becomes:</p> <pre><code>b*(abb*)*a? </code></pre> <p>In python we would write:</p> <pre><code>p = re.compile(r'^b*(abb*)*a?$') </code></pre></li> <li><p>The reason straight translation of regular regular expression syntax into python (i.e. copy and paste) does not work is because python matches the <em>shortest substring</em> (if the symbols $ or ^ are absent) while the theoretical regular expressions match <em>longest initial substring</em>. <br/> So for example if we had a string:</p> <pre><code>s = 'aa' </code></pre> <p>Our textbook regex <strong>b*(abb*)*a?</strong> would not match it because it has two a's. However if we copy it straight to python:</p> <pre><code>>> p = re.compile(r'b*(abb*)*a?') >> bool(p.match(s)) True </code></pre> <p>This is because our regex matches only the substring 'a' of our string 'aa'. <br/> In order to tell python to do a match on the whole string we have to tell it where the beginning and the end of the string is, with the <strong>^</strong> and <strong>$</strong> symbols respectively:</p> <pre><code>>> p = re.compile(r'^b*(abb*)*a?$') >> bool(p.match(s)) False </code></pre> <p>Note that python regex <strong>match()</strong> matches at the <em>beginning</em> of the string, so it automatically assumes the <strong>^</strong> at the start. However the <strong>search()</strong> function does not, and thus we keep the <strong>^</strong>.<br/> So for example:</p> <pre><code>>> s = 'aa' >> p = re.compile(r'b*(abb*)*a?$') >> bool(p.match(s)) False # Correct >> bool(p.search(s)) True # Incorrect - search ignored the first 'a' </code></pre></li> </ol>

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