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<font class="title2"><b>练习</b></font>
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4.1 考虑文法 <br>
R→R'|'R|RR|R*|(R)|a|b </p>
<p>请注意第一个直竖是符号“或”，而不是两个候选式之间的分隔符号。<br>
(1)试说明此文法生成在符号a和b之上的所有正规表达式(除Φ，ε)。 <br>
(2)试说明此文法是二义性的。<br>
(3)试构造一个等价的无二义性文法，此文法给出*、连接和|等运算符号的优先级和结合规则，有关约定见3．4节。<br>
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<a name="4.2"></a>
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4.2 试分析下面给出的if-then-else语句的文法，它的提出原本是为了矫正dangling-else (悬而未决的-else)文法的二义性：<br>
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<tr><td>stmt →</td><td>if expr then stmt </td></tr>
<tr><td></td><td>|matched-stmt</td></tr>
<tr><td>matched-stmt→</td><td>if expr then matched-stmt else stmt </td></tr>
<tr><td></td><td>|other </td></tr>
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试说明此文法仍然是二义性的。 <br>
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<a name="4.3"></a>
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4.3 对于给定文法G[bexpr]，其产生式如下： <br>
bexpr→bexpr or bterm | bterm <br>
bterm→bterm and bfactor | bfactor <br>
bfactor→not bfactor|(bexpr) |true |false <br>
构造一个递归预测分析程序<br>
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<a name="4.4"></a>
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4.4 已知文法G[S] <br>
S→(L)|a <br>
L→ L，S|S <br>
试为文法G[S]构造一个递归的预测分析器。<br>
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