Question

Can I specify I want the preceding character to be either (NOT A DIGIT) or (BEGINNING ANCHOR)?

I’m looking for “1/2”, but I want to ignore cases like 21/22. I know I can rule out preceding or trailing digits like this:

/([^0-9])1\/2([^0-9])/

But that fails when the match happens at the beginning or end of the line. Are anchors ^ and & allowed in grouping clauses?

Answer 1

If you also want to match something like a1/2, you can use a negative lookbehind (provided that your regex implementation supports it):

irb(main):015:0> 'a1/2' =~ /(?<!\d)1\/2/
=> 1
irb(main):016:0> '11/2' =~ /(?<!\d)1\/2/
=> nil

Which is “the string 1/2, not preceded by a digit”.

If you can’t use lookbehind, you can almost literally transfer your requirements into a regex like /([^\d]|^)(1\/2)/, which is “a character that is not a digit or the beginning of the phrase, followed by the string 1/2 (second capture group)”.