In my previous post, one of the examples I used of when capturing groups are appropriate demonstrated how to match quoted strings:
(Note: This has some issues. See Edit 2, at the end of this post, if you intend to use this in an application.)
To recap, that will match values enclosed in either double or single quotes, while requiring that the same quote type start and end the match. It also allows inner, escaped quotes of the same type as the enclosure.
On his blog, Ben Nadel asked:
I do not follow the
\\\1in the middle group. You said that that was an escaped closing of the same type (group 1). I do not follow. Does that mean that the middle group can have quotes in it? If that is the case, how does the reluctant search in the middle (
*?) know when to stop if it can have quotes in side of it? What am I missing?
Good question. Following is the response I gave, slightly updated to improve clarity:
First, to ensure we're on the same page, here are some examples of the kinds of quoted strings the regex will correctly match:
In other words, it allows any number of escaped quotes of the same type as the enclosure.
As for how the regex works, it uses a trick similar in construct to the examples I gave in my blog post about regex recursion.
Basically, the inner grouping matches escaped quotes OR any single character, with the escaped quote part before the dot in the test attempt sequence. So, as the lazy repetition operator (
*?) steps through the match looking for the first closing quote, it jumps right past each instance of the two characters which together make up an escaped quote. In other words, pairing something other than the quote mark with the quote mark makes the lazy repetition operator treat them as one node, and continue on it's way through the string.
(["'])(?:\\\1|.)*?\1(the regex being discussed)
(["']).*?(?<!\\)\1(uses a negative lookbehind to achieve logic which is possibly simpler to understand)
One final thing worth noting is that in neither regex did I try to use anything like
[^\1] for matching the inner, quoted content. If
[^\1] worked as you might expect, it might allow us to construct a slightly faster regex which would greedily jump from the start to the end of each quote and/or between escaped quotes. First of all, the reason we can't greedily repeat an "any character" pattern such as a dot or
[\S\s] is that we would then no longer be able to distinguish between multiple discrete quotes within the same string, and our match would go from the start of the first quote to the end of the last quote. The reason we can't use
[^\1] either is because you can't use backreferences within character classes, even though in this case the backreference's value is only one character in length. Also note that the patterns
[^\1] actually do have special meaning, though possibly not what you would expect. With most regular expression libraries, they assert: match a single character which is/is not octal index 1 in the character set. To assert that outside of a character class, you'd typically need to use a leading zero (e.g.,
\01), but inside a character class the leading zero is often optional.
If anyone has questions about how or why other specific regex patterns work or don't work, let me know, and I can try to make "Regexes in Depth" a regular feature here.
Edit: Just for kicks, here's a version which adds support for fancy, angled “…” and ‘…’ pairs. This uses a lookbehind and conditionals. Libraries which support both features include the .NET framework and PCRE.
In the above, I'm using nested conditionals to achieve an if/elseif/else construct. Here are some examples of the kinds of quoted strings the above regex adds support for (in addition to preserving support for quotes enclosed with " or '.
Edit 2: Note that these regexes were designed more for illustrative purposes than practical use within programming. One issue is that they don't account for escaped backslashes within the quotes (e.g., they treat \\" as a backslash followed by an escaped double quote, rather than an escaped backslash followed by an unescaped double quote. However, that's easy to address. For the first regex, just replace it with this:
To also avoid an issue with quote marks which are followed by an escaped quote of the same type but are not followed by a closing quote, make the first quantifier possessive, like so:
Or, if the regex engine you're using doesn't support possessive quantifiers or atomic groups (which can be used equivalently), use one of the following:
The former mimics an atomic group, and the later utilizes a negative lookahead which allows replacing the lazy star with a greedy one. There is still potential to optimize these for efficiency, and none of them account for the outermost opening quote marks being escaped or other issues regarding context (e.g., you might want to ignore quoted strings within code comments), but still, these are probably good enough for most people's needs.