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| -rw-r--r-- | ChangeLog | 1 | ||||
| -rw-r--r-- | txr.1 | 18 |
2 files changed, 13 insertions, 6 deletions
@@ -3,6 +3,7 @@ * txr.1: Get rid of parens from regex operator descriptions. Correct wrong text: all operators can take an empty regex. Clarify escaping rules within a character class. + Describe Kleene and non-greedy behavior more accurately. 2010-01-15 Kaz Kylheku <kkylheku@gmail.com> @@ -662,13 +662,19 @@ optionally match the preceding regular expression R. .IP R+ match the preceding expression R one or more times, as many times as possible. .IP R* -match the expression R zero or more times, as many times as possible. This -operator is sometimes called the "Kleene operator" or "Kleene star". +match the expression R zero or more times. This +operator is sometimes called the "Kleene star", or "Kleene closure". +The Kleene closure favors a longest match. Roughly speaking, if there are two +or more ways in which R1*R2 can match, that that match occurs in which +R1* matches the longest possible text. .IP R1%R2 -match R1 zero or more times, but not as many times as possible: stop the -match at the first point where R2 matches, even if repetitions of R1 can -continue matching. This is called the non-greedy operator. R2 may be -an empty regular expression, in which case this is equivalent to R1*. +match R1 zero or more times, then match R2. If this match can occur in +more than one way, then it occurs such that R1 is matched the fewest +number of times; which is opposite from the behavior of R1*R2. +In other words, repetitions of R1 terminate at the earliest +point in the text where a match for R2 occurs. Favoring shorter matches, % is +termed a non-greedy operator. Note that R2 may be an empty regular expression, +which is a special case that is equivalent to R1*. .IP ~R match the complement of the following expression R; i.e. match those texts that R does not match. This operator is called complement, |