Re^2: negate pattern match

Actually this brings me to another question along the same lines which is...if I wanted to (as I do) check for records where there may be a keyword before "train times" that may mean the record is to be rejected i.e. a negative lookbehind assertion I assume
e.g.

help train times from xyz
load train times including xyz
help train times including xyz
book train times at 1234
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I need to reject records where the word immediately before "train times" is help. If help is used in the text it would always be the first word in the record. So only record two & four in my example would be printed. I tried doing this

print if /(?<!help).*train times/; #doesn't work
print if /^(!help).*train times/; # doesn't work either
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Based on other comments I've read it seems that the use of negative lookbehind assertion should be discouraged but I'm not sure how to handle this without doing an assertion ?

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Re^3: negate pattern match by GrandFather (Saint) on Jan 31, 2006 at 18:39 UTC
A negative look ahead assertion at the start of the line (not a look back) is what you want: `use strict; use warnings; while (<DATA>) { print "Matched: $_" if /^(?!help).*?train times (?!to\b\|from\b)\w/; } __DATA__ help train times from xyz load train times including xyz help train times including xyz book train times at 1234` [download] Prints: `Matched: load train times including xyz Matched: book train times at 1234` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^4: negate pattern match by Anonymous Monk on Feb 02, 2006 at 10:56 UTC
Thanks. I never realised how much there are to ascertians !	[reply]