perl REGEX -- What does it mean that regexes are greedy? How can I get around it?
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From: xlr82sas on 8 Mar 2010 21:38 6.13: What does it mean that regexes are greedy? How can I get around it? Most people mean that greedy regexes match as much as they can. Technically speaking, it's actually the quantifiers ("?", "*", "+", "{}") that are greedy rather than the whole pattern; Perl prefers local greed and immediate gratification to overall greed. To get non- greedy versions of the same quantifiers, use ("??", "*?", "+?", "{}?"). An example: $s1 = $s2 = "I am very very cold"; $s1 =~ s/ve.*y //; # I am cold $s2 =~ s/ve.*?y //; # I am very cold Notice how the second substitution stopped matching as soon as it encountered "y ". The "*?" quantifier effectively tells the regular expression engine to find a match as quickly as possible and pass control on to whatever is next in line, like you would if you were playing hot potato. In SAS Note without the '?' perl is greedy and takes out both 'very' words, Even when I have 1 specified. With the ? it takes out the first 'very'. Note we have another option in SAS to take out 'n; 'very's. see below data _null_; s1="I am very very cold"; o1=prxchange('s/ver.*y //',1,s1); o2=prxchange('s/ver.*?y //',1,s1); o3=prxchange('s/ver.*?y //',2,s1); put (o1 o2 o3) (/=); run; 30874 data _null_; 30875 s1="I am very very cold"; 30876 o1=prxchange('s/ver.*y //',1,s1); 30877 o2=prxchange('s/ver.*?y //',1,s1); 30878 o3=prxchange('s/ver.*?y //',2,s1); 30879 put (o1 o2 o3) (/=); 30880 run; O1=I am cold O2=I am very cold O3=I am cold NOTE: DATA statement used (Total process time): real time 0.00 seconds cpu time 0.00 seconds |