Regex not to capture if a string contains a word before it

Question

I want to select the word "hazardous" only if it is a separate word and not with "non " or "non-"before it.

eg:

non-hazardous

non hazardous

hazardous

non agricultural hazardous

regex 1: ^(?!non[-/s]?)hazardous$
regex 2: ^(?!non-|non\s)hazardous$

I tried the above two regex and it gave correct results for the first 3 sentences, but it's not selecting hazardous in 4th sentence. I want to select hazardous in 4th sentence as it doesn't have "non " or "non-" before it

Reference: Regular Expression - Match pattern that does not contain a string

Maybe just r'\b(?<!\bnon[-\s])hazardous\b'? See regex101.com/r/qmGLvQ/1 — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Mar 10, 2022 at 18:21

Wiktor Stribiżew · Accepted Answer · 2022-03-10 18:24:26Z

2

You can use

r'\b(?<!\bnon[-\s])hazardous\b'

See the regex demo. The pattern matches

\b - a word boundary
(?<!\bnon[-\s]) - a negative lookbehind that fails the match if there is non- or non and a whitespace immediately to the left of the current location
hazardous - a string
\b - a word boundary.

answered Mar 10, 2022 at 18:24

Wiktor Stribiżew

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6 Comments

ConMan77 Over a year ago

why is there a word boundary in front of everything? Can you explain in detail

Wiktor Stribiżew Over a year ago

@ConMan77 In order to match the whole word.

ConMan77 Over a year ago

(?<!\bnon-|non\s)hazardous\b can this be used ?

Wiktor Stribiżew Over a year ago

@ConMan77 Yes if it works for you, but the alternation inside an unanchored lookbehind is resource consuming. Using character classes is more efficient when possible,

Cary Swoveland Over a year ago

I often see \s used in place of a space where the only relevant whitespace character is a space. Your regex does not match "One is non\nhazardous", but arguably should. That's a contrived example, of course (and here it's hard to see how the use of \s could cause a problem), but it seems to me that, as a rule, one should always use a space rather than \s when a space is the only whitespace character that is to be matched or not matched. Your opinion?

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