Question

As stated in the title, is there a way, using regular expressions, to match a text pattern for text that appears outside of quotes. Ideally, given the following examples, I would want to be able to match the comma that is outside of the quotes, but not the one in the quotes.

This is some text, followed by ‘text, in quotes!’

or

This is some text, followed by ‘text, in quotes’ with more ‘text, in quotes!’

Additionally, it would be nice if the expression would respect nested quotes as in the following example. However, if this is technically not feasible with regular expressions then it wold simply be nice to know if that is the case.

The programmer looked up from his desk, ‘This can’t be good,’ he exclaimed, ‘the system is saying ‘File not found!”

I have found some expressions for matching something that would be in the quotes, but nothing quite for something outside of the quotes.

Answer 1

This can be done with modern regexes due to the massive number of hacks to regex engines that exist, but let me be the one to post the ‘Don’t Do This With Regular Expressions’ answer.

This is not a job for regular expressions. This is a job for a full-blown parser. As an example of something you can’t do with (classical) regular expressions, consider this:

()(())(()()) 

No (classical) regex can determine if those parenthesis are matched properly, but doing so without a regex is trivial:

/* C code */  char string[] = '()(())(()())'; int parens = 0; for(char *tmp = string; tmp; tmp++) {   if(*tmp == '(') parens++;   if(*tmp == ')') parens--; } if(parens > 0) {   printf('%s too many open parenthesis.\n', parens); } else if(parens < 0) {   printf('%s too many closing parenthesis.\n', -parens); } else {   printf('Parenthesis match!\n'); }  # Perl code  my $string = '()(())(()())'; my $parens = 0; for(split(//, $string)) {   $parens++ if $_ eq '(';   $parens-- if $_ eq ')'; } die 'Too many open parenthesis.\n' if $parens > 0; die 'Too many closing parenthesis.\n' if $parens < 0; print 'Parenthesis match!'; 

See how simple it was to write some non-regex code to do the job for you?

EDIT: Okay, back from seeing Adventureland. 🙂 Try this (written in Perl, commented to help you understand what I’m doing if you don’t know Perl):

# split $string into a list, split on the double quote character my @temp = split(/'/, $string);  # iterate through a list of the number of elements in our list for(0 .. $#temp) {    # skip odd-numbered elements - only process $list[0], $list[2], etc.   # the reason is that, if we split on 's, every other element is a string   next if $_ & 1;    if($temp[$_] =~ /regex/) {     # do stuff   }  } 

Another way to do it:

my $bool = 0; my $str; my $match;  # loop through the characters of a string for(split(//, $string)) {    if($_ eq ''') {     $bool = !$bool;     if($bool) {        # regex time!       $match += $str =~ /regex/;        $str = '';     }   }    if(!$bool) {      # add the current character to our test string     $str .= $_;   } }  # get trailing string match $match += $str =~ /regex/; 

(I give two because, in another language, one solution may be easier to implement than the other, not just because There’s More Than One Way To Do It™.)

Of course, as your problems grow in complexity, there will arise certain benefits of constructing a full-blown parser, but that’s a different horse. For now, this will suffice.