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I've created a regex which I need to run with grep, I'm pretty sure the regex is fine as it works with online regex tools, however when I run

grep -r -P -o -h '(?<=(?<!def )my_method )(["'])(?:(?=(\\?))\2.)*?\1'

I get the error Syntax error: ")" unexpected.

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Your regular expression is quoted with single quotes, but it also contains a single quote.

The single quote in ["'] needs to be escaped, or it will signal the end of the quoted string to the shell.

This will fix it:

grep -r -P -o -h '(?<=(?<!def )my_method )(["'\''])(?:(?=(\\?))\2.)*?\1'
#                                            ^^^^

With ["'\''], the first ' ends the first part of the string, the \' inserts a literal single quote, and the last ' starts a new single quoted string that will be concatenated with the previous bits. Only the middle single quote will end up in the regular expression itself, the other two will be removed by the shell.

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    Nice Tricky method. Alternative : (["\x27]) will work fine (replacing the single quote with it's ascii code) Apr 3 '17 at 12:23
  • @GeorgeVasiliou That's also a good solution, yes. \x27 is as many characters as '\''.
    – Kusalananda
    Apr 3 '17 at 12:24
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    Though '\'' is transferable to other non-regex situations whereas \x27 is not.
    – Muzer
    Apr 3 '17 at 14:41
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As @Kusalananda explained, the problem is the ' inside the regex. A simple solution is to use " for the regex since " can be escaped even inside a "-quoted string, unlike ' which cannot be escaped inside a '-quoted string:

grep -rPoh "(?<=(?<!def )my_method )([\"'])(?:(?=(\\?))\2.)*?\1"

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