(libc.info.gz) Subexpression Complications

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(libc.info.gz) Regexp Subexpressions

(libc.info.gz) Regular Expressions

(libc.info.gz) Regexp Cleanup

 
 10.3.5 Complications in Subexpression Matching
 ----------------------------------------------
 
 Sometimes a subexpression matches a substring of no characters.  This
 happens when 'f\(o*\)' matches the string 'fum'.  (It really matches
 just the 'f'.)  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 '1'.
 
    Sometimes the entire regular expression can match without using some
 of its subexpressions at all--for example, when 'ba\(na\)*' matches the
 string 'ba', the parenthetical subexpression is not used.  When this
 happens, 'regexec' stores '-1' in both fields of the element for that
 subexpression.
 
    Sometimes matching the entire regular expression can match a
 particular subexpression more than once--for example, when 'ba\(na\)*'
 matches the string 'bananana', the parenthetical subexpression matches
 three times.  When this happens, 'regexec' usually stores the offsets of
 the last part of the string that matched the subexpression.  In the case
 of 'bananana', these offsets are '6' and '8'.
 
    But the last match is not always the one that is chosen.  It's more
 accurate to say that the last _opportunity_ 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 '\(ba\(na\)*s \)*' matching the
 string 'bananas bas '.  The last time the inner expression actually
 matches is near the end of the first word.  But it is _considered_ again
 in the second word, and fails to match there.  'regexec' reports nonuse
 of the "na" subexpression.
 
    Another place where this rule applies is when the regular expression
      \(ba\(na\)*s \|nefer\(ti\)* \)*
 matches 'bananas nefertiti'.  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 'regexec' reports
 nonuse of the "na" subexpression.
 

Info Catalog

(libc.info.gz) Regexp Subexpressions

(libc.info.gz) Regular Expressions

(libc.info.gz) Regexp Cleanup