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| <h4 class="subsection">10.3.5 Complications in Subexpression Matching</h4> |
| |
| <p>Sometimes a subexpression matches a substring of no characters. This |
| happens when ‘<samp><span class="samp">f\(o*\)</span></samp>’ matches the string ‘<samp><span class="samp">fum</span></samp>’. (It really |
| matches just the ‘<samp><span class="samp">f</span></samp>’.) In this case, both of the offsets identify |
| the point in the string where the null substring was found. In this |
| example, the offsets are both <code>1</code>. |
| |
| <p>Sometimes the entire regular expression can match without using some of |
| its subexpressions at all—for example, when ‘<samp><span class="samp">ba\(na\)*</span></samp>’ matches the |
| string ‘<samp><span class="samp">ba</span></samp>’, the parenthetical subexpression is not used. When |
| this happens, <code>regexec</code> stores <code>-1</code> in both fields of the |
| element for that subexpression. |
| |
| <p>Sometimes matching the entire regular expression can match a particular |
| subexpression more than once—for example, when ‘<samp><span class="samp">ba\(na\)*</span></samp>’ |
| matches the string ‘<samp><span class="samp">bananana</span></samp>’, the parenthetical subexpression |
| matches three times. When this happens, <code>regexec</code> usually stores |
| the offsets of the last part of the string that matched the |
| subexpression. In the case of ‘<samp><span class="samp">bananana</span></samp>’, these offsets are |
| <code>6</code> and <code>8</code>. |
| |
| <p>But the last match is not always the one that is chosen. It's more |
| accurate to say that the last <em>opportunity</em> to match is the one |
| that takes precedence. What this means is that when one subexpression |
| appears within another, then the results reported for the inner |
| subexpression reflect whatever happened on the last match of the outer |
| subexpression. For an example, consider ‘<samp><span class="samp">\(ba\(na\)*s \)*</span></samp>’ matching |
| the string ‘<samp><span class="samp">bananas bas </span></samp>’. The last time the inner expression |
| actually matches is near the end of the first word. But it is |
| <em>considered</em> again in the second word, and fails to match there. |
| <code>regexec</code> reports nonuse of the “na” subexpression. |
| |
| <p>Another place where this rule applies is when the regular expression |
| <pre class="smallexample"> \(ba\(na\)*s \|nefer\(ti\)* \)* |
| </pre> |
| <p class="noindent">matches ‘<samp><span class="samp">bananas nefertiti</span></samp>’. The “na” subexpression does match |
| in the first word, but it doesn't match in the second word because the |
| other alternative is used there. Once again, the second repetition of |
| the outer subexpression overrides the first, and within that second |
| repetition, the “na” subexpression is not used. So <code>regexec</code> |
| reports nonuse of the “na” subexpression. |
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