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Greedy, reluctant, and possessive quantifiers

You can read this regular expression as “zero or one occurrence of
b
, followed by
r
, followed by
e
, followed
by zero or one occurrence of
a
, followed by
d
.” The preceding regular expression is the same as this one:
In this regular expression, the question mark has been replaced with curly braces. Inside the curly braces
are the numbers 0, which is the minimum number of occurrences, and 1, which is the maximum. This
expression reads the same way as the previous one; it’s just represented differently. Both expressions are
considered correct.
To illustrate the other quantifiers, suppose you had to create a regular expression to match the strings
words each match.
Regular Expression
Matches
ba?d
ba*d
ba+d
ba{0,1}d
ba{0,}d
ba{1,}d
As you can see, only two of the six expressions adequately solve the problem:
ba*d
and
ba{0,}d
. Notice
that these two are exactly equal because the asterisk means
0 or more
just as
{0,}
does. Likewise, the first
and fourth expressions are equal, and the third and sixth expressions are equal.
Quantifiers can also be used with character classes, so if you wanted to match the strings “bead”,
“baed”, “beed”, “baad”, “bad”, and “bed”, the following regular expression would do so:
This expression says that the character class
[ae]
can appear a minimum of one time and a maximum
of two times.
Greedy, reluctant, and possessive quantifiers
The three kinds of regular expression quantifiers are
greedy
,
reluctant
, and
possessive
.
A
greedy quantifier
starts by looking at the entire string for a match. If no match is found, it eliminates
the last character in the string and tries again. If a match is still not found, the last character is again
discarded and the process repeats until a match is found or the string is left with no characters. All the
quantifiers discussed to this point have been greedy.
A
reluctant quantifier
starts by looking at the first character in the string for a match. If that character
alone isn’t enough, it reads in the next character, forming a string of two characters. If still no match is
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Chapter 7
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