I am looking for expressions as Vc Am in texts and for that I have

rex = r"(\(?)(?<!([A-Za-z0-9]))[A-Z][a-z](?!([A-Za-z0-9]))(\)?)"

explanation:

[A-Z][a-z] = Cap followed by lower case letter 
(?<!([A-Za-z0-9])) -> lookbehind not being a letter or number
(?!([A-Za-z0-9]))(\)?) _ Look ahead not being letter or number
# all that optionally wihtin parenthesis

import re
text="this is Vc and not Cr nor Pb"
matches = re.finditer(rex,text)

What I want to achieve is exclude a list of terms like Cr or Pb.

How should I include exceptions in the expression?

thanks

CodePudding user response：

First, let's shorten your RegEx:

• (?<!([A-Za-z0-9])) -> lookbehind not being a letter or number
• (?!([A-Za-z0-9]))(\)?) -> look ahead not being letter or number

these are so common there is a RegEx feature for them: Word boundaries \b. They have zero width like lookarounds and only match if there is no alphanumeric character.

Your RegEx then becomes \b[A-Z][a-z]\b; looking at this RegEx (and your examples), it appears you want to match certain element abbreviations?

Now you can simply use a lookbehind:

\b[A-Z][a-z](?<!Cr|Pb)\b

to assert that the element is neither Chrome nor Lead.

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Just for fun:

Alternatively, if you want a less readable (but more portable) RegEx that makes do with fewer advanced RegEx features (not every engine supports lookaround), you can use character sets as per the following observations:

• If the first letter is not a C or P, the second letter may be any lowercase letter;
• If the first letter is a C, the second letter may not be an r
• If the first letter is a P, the second letter may not be an b

Using character sets, this gives us:

• [ABD-OQ-Z][a-z]
• C[a-qs-z]
• P[ac-z]

Operator precedence works as expected here: Concatenation (implicit) has higher precendence than alteration (|). This makes the RegEx [ABD-OQ-Z][a-z]|C[a-qs-z]|P[ac-z]. Wrapping this in word boundaries using a group gives us \b([ABD-OQ-Z][a-z]|C[a-qs-z]|P[ac-z])\b.

CodePudding user response：

You might write the pattern without using the superfluous capture groups, and exclude matching Cr or Pb:

\(?(?<![A-Za-z0-9])(?!Cr\b|Pb\b)[A-Z][a-z](?![A-Za-z0-9])\)?

See a regex demo for the matches.

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If you are not interested in matching the parenthesis, and you also do not want to allow an underscore along with the letters or numbers, you can use a word boundary instead:

\b(?!Cr\b|Pb\b)[A-Z][a-z]\b

Explanation

• \b A word boundary to prevent a partial word match
• (?! Negative lookahead
  • Cr\b|Pb\b Match either Cr or Pb
• ) Close the lookahead
• [A-Z][a-z] Match a single uppercase and single lowercase char
• \b A word boundary

Regex demo