Regex lookaround does not work with quantifiers in SAS

user_9021547 0
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I have a table similar to this:

Data have;
text = 'insurance premium'; output;
text = 'insur. premium'; output;
text = 'premium. insur aa'; output;
text = 'premium card'; output;
text = 'sales premium'; output;
Run;

My task is to select all transactions that contain the word premium, but do not contain the word insurance or a form thereof (e.g. insur, ins. etc.). I read up on how to use lookaround expressions in regex and wrote the following expression:

/(?<!ins[a-z.]*\s)premium(?!.*ins[a-z.]*\s)/i

The expression seems to work on testing websites such as https://regexr.com/, but when I run the code below I get an error in SAS:

Data want;
Set have;
re = prxparse('/(?<!ins[a-z.]*\s)premium(?!.*ins[a-z.]*\s)/i');
flg = prxmatch(re, text) > 0;
Run;

ERROR: Variable length lookbehind not implemented before HERE mark in regex m/(?<!insur[a-z.]*\s)premium(?!.*insur[a-z.]*\s) << 
       HERE /.
ERROR: Variable length lookbehind not implemented before HERE mark in regex m/(?<!ins[a-z.]*\s)premium(?!.*ins[a-z.]*\s) << HERE /.

ERROR: The regular expression passed to the function PRXPARSE contains a syntax error.
NOTE: Argument 1 to function PRXPARSE('/(?<!ins[a-z'[12 of 45 characters shown]) at line 30 column 6 is invalid.
NOTE: Argument 1 to the function PRXMATCH is missing.

As far as I understood there is an issue with the * symbols inside the lookaround functions, because the error does not occur if I remove them. Does SAS implement such expressions differently or does it simply not support such expressions?

sas regex-lookarounds
2 Answers

You cannot use a lookbehind with a variable width pattern in a PCRE regex. However, you can match and skip substrings you do not need using (*SKIP)(*FAIL) verbs, so you can revamp the regex you have in the following way:

prxparse('/ins[a-z.]*\spremium(?!.*ins[a-z.]*\s)(*SKIP)(*F)|premium(?!.*ins[a-z.]*\s)/i')

Mind that patterns are parsed and searched for from left to right. ins[a-z.]*\spremium(?!.*ins[a-z.]*\s)(*SKIP)(*F)| is triggered first, and if ins[a-z.]*\spremium(?!.*ins[a-z.]*\s) is found, it is skipped. Else, the second premium(?!.*ins[a-z.]*\s) alternative comes into play and matches premium not followed with ins and zero or more letters / dots and a whitespace in other contexts.

user_38329700

You are using flg = prxmatch(re, text) > 0; to see if there is a match by checking if the position is > 0

You can put the negative lookahead at the start of the string to check for the variations of insurance, and then match the word premium.

^(?!.*\bins[a-z.]*\s).*\bpremium\b

Explanation

  • ^ Start of string
  • (?! Negative lookahead, assert that on the right is not
    • .*\bins Match a word starting with ins
    • [a-z.]*\s Optionally repeat matching chars a-z or . followed by a whitespace char
  • ) Close the lookahead
  • .*\bpremium\b match the word premium in the line

Regex demo

user_54249880
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