Question

Is there a way to back reference in the regular expression pattern?

Example input string:

Here is "some quoted" text.

Say I want to pull out the quoted text, I could create the following expression:

"([^"]+)"

This regular expression would match some quoted.

Say I want it to also support single quotes, I could change the expression to:

["']([^"']+)["']

But what if the input string has a mixture of quotes say Here is 'some quoted" text. I would not want the regex to match. Currently the regex in the second example would still match.

What I would like to be able to do is if the first quote is a double quote then the closing quote must be a double. And if the start quote is single quote then the closing quote must be single.

Can I use a back reference to achieve this?

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My other related question: Getting text between quotes using regular expression

Answer 1

You can make use of the regex:

(["'])[^"']+\1

• () : used for grouping
• [..] : is the char class. so ["']
  matches either " or ' equivalent
  to "|'
• [^..] : char class with negation.
  It matches any char not listed after
  the ^
• + : quantifier for one or more
• \1 : backreferencing the first
  group which is (["'])

In PHP you’d use this as:

preg_match('#(["\'])[^"\']+\1#',$str)