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PONon-greedy Pattern Expression
 

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    <p>I've been reading "Mastering Regular Expressions" by Friedl and trying to devise a common non-greedy pattern expression for a string that is delimited by a word. Starting from basics where the delimited word is just a single character '<strong>a</strong>' the expression:</p> <pre><code>sed -r 's/([^a]*)(a)/\ (1)\1(2)\2(ALL)&amp;(END)/g' &lt;&lt;&lt;"xaxxaxxxaxxx...aa..." (1)x(2)a(ALL)xa(END) (1)xx(2)a(ALL)xxa(END) (1)xxx(2)a(ALL)xxxa(END) (1)xxx...(2)a(ALL)xxx...a(END) (1)(2)a(ALL)a(END)... </code></pre> <p>from which the pattern (with reference to Friedl) might be:</p> <ul> <li>[ <strong><em>normal* closing</em></strong> ]</li> </ul> <p>Moving on to a real multi-character '<strong>ab</strong>' delimiter:</p> <pre><code>sed -r 's/([^a]*)((a[^b]*)*)(ab)/\ (1)\1(2)\2(3)\3(4)\4(ALL)&amp;(END)/g' &lt;&lt;&lt;"xabxxabxxxabxxx...abxxx...aabxxx...axxx...aaabxaabaxabaxaxabxaxaabxxaaabaaxxab..." (1)x(2)(3)(4)ab(ALL)xab(END) (1)xx(2)(3)(4)ab(ALL)xxab(END) (1)xxx(2)(3)(4)ab(ALL)xxxab(END) (1)xxx...(2)(3)(4)ab(ALL)xxx...ab(END) (1)xxx...(2)a(3)a(4)ab(ALL)xxx...aab(END) (1)xxx...(2)axxx...aa(3)axxx...aa(4)ab(ALL)xxx...axxx...aaab(END) (1)x(2)a(3)a(4)ab(ALL)xaab(END) (1)(2)ax(3)ax(4)ab(ALL)axab(END) (1)(2)axax(3)axax(4)ab(ALL)axaxab(END) (1)x(2)axa(3)axa(4)ab(ALL)xaxaab(END) (1)xx(2)aa(3)aa(4)ab(ALL)xxaaab(END) (1)(2)aaxx(3)aaxx(4)ab(ALL)aaxxab(END)... </code></pre> <p>from which the pattern might be:</p> <ul> <li>[ <strong><em>normal* (special*)* closing</em></strong> ] </li> </ul> <p>For the subsequent '<strong>abc</strong>' delimiter the <strong><em>special</em></strong> expression can be extended to:</p> <pre><code>(a[^b]*)*(ab[^c]*)* </code></pre> <ol> <li>Is this correct?</li> <li>Can it be proved?</li> <li>Can the <strong><em>special</em></strong> expression be simplified?</li> <li>Are there better/more efficient expressions for this? n.b. I'm not using perl's non-greedy '*?' operator and avoiding alternation.</li> <li>Where might I find reference material to this type of problem (Friedl alluded but stopped short of a published solution).</li> </ol>
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    1. COI'm so far off from "Mastering Regular Expressions" it's not even funny. I'm interrested however. Would you care to explain why you don't want to use those two operators: ? and |. Appreciate it.
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    2. COWhy not use negative lookahead ?
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    3. CO@Ikuty I'm afraid that's not part of sed's reportoire.
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