Regular expression: matching words between white space

Question

Im trying to do something fairly simple with regular expression in python... thats what i thought at least.

What i want to do is matching words from a string if its preceded and followed by a whitespace. If its at the beginning of the string there is no whitespace required before - if its at the end, dont't search for whitespace either.

Example:

"WordA WordB WordC-WordD WordE"

I want to match WordA WordB WordE.

I only came up with overcomplicated way of doing this...

(?<=(?<=^)|(?<=\s))\w+(?=(?=\s)|(?=$))

It seems to me there has to be a simple way for such a simple problem.... I figured i can just start with (?<=\s|^) but that doesnt seem possible because "look-behind requires fixed-width pattern".

Commutate

Answer 1

You seem to work in Python as (?<=^|\s) is perfectly valid in PCRE, Java and Ruby (and .NET regex supports infinite width lookbehind patterns).

Use

(?<!\S)\w+(?!\S)

It will match 1 or more word chars that are enclosed with whitespace or start/end of string.

See the regex demo.

Pattern details:

• (?<!\S) - a negative lookbehind that fails the match once the engine finds a non-whitespace char immediately to the left of the current location
• \w+ - 1 or more word chars
• (?!\S) - a negative lookahead that fails the match once the engine finds a non-whitespace char immediately to the right of the current location.

Preparator

that makes sense! Thanks. I guess searching for nonwhitespace instead of whitespace is much easier. – Commutate

Not sure it is easier, but is more efficient. – Apc

I don't understand why simply \s+ surrounding what we need does not work – Belda

@BFurtado Because \s consumes a whitespace. Look at this demo: there is only one match because the \s on both ends requires a whitespace on the left and right. WordA and WordE have no whitespace on one end. You might think (\s|^)\w+(\s|$) will work, but it does not match consecutive occurrences because (\s|$) consumes the whitespace after WordA and thus (\s|^) cannot find the WordB match. – Apc

Thank you very much @WiktorStribiżew. I have struggled with regex countless times. Official documentation docs.python.org/3/howto/regex.html says nothing about consuming space. There is rather cryptic mention of zero-width (which seems out of context for me, but may resemble what you are kindly explaining. Best, – Belda

@BFurtado I will try to explain it in my Youtube channel and share a link with you (the channel link is in my profile). – Apc