grep command with a lookahead pattern does not select anything

Question

I was trying to use the following grep command:

grep \'(.*)(?=(png|html|jpg|js|css)(?:\\s*))(png|html|jpg|js|css.*\\s)\' file

File contains the

Answer 1

You are using a PCRE regex with the POSIX BRE engine that is default grep engine.

To make those patterns work, you should use -P option (available in GNU grep):

grep -P 'YOUR_PCRE_PATTERN'
     ^^

To develop and test PCRE patterns, a well-known regex101.com is usually recommended.

Note that on Mac OS, you may install GNU grep via brew.