Question

Given a textarea, similar to StackOverflow, I’d like to wrap code (indented by 4 spaces) with a pre/code block. I’m trying to use the following regex to find the code:

re = / # Match a MARKDOWN CODE section.
    (\r?\n)              # $1: CODE must be preceded by blank line
    (                    # $2: CODE contents
      (?:                # Group for multiple lines of code.
        (?:\r?\n)+       # Each line preceded by a newline,
        (?:[ ]{4}|\t).*  # and begins with four spaces or tab.
      )+                 # One or more CODE lines
      \r?\n              # CODE folowed by blank line.
    )                    # End $2: CODE contents
    (?=\r?\n)            # CODE folowed by blank line.
    /x
result = subject.gsub(re, '\1<pre>\2</pre>')

But this isn’t working, here’s the example in Rubular:

http://rubular.com/r/l5faSjR8ya

Any suggestions on how to have this Regex, match the code allow me to wrap a pre/code tags around the code? Thanks

Answer 1

I think there is an escape out of the code mode with any trailing newline not followed by tab or 4 spaces. Not sure but successive newlines would not be included in the code block.

I don’t get Ruby’s regex options too well, but this seems to work: http://rubular.com/r/BlbreoO3sn

((?:^(?:[ ]{4}|\t).*$(?:\r?\n|\z))+) Theorhetically, its in multi-line mode.

Just make the replacement <pre>\1</pre>

EDIT
@Rachela Meadows – After further examination, this is a fairly difficult regex.
I managed to exactly duplicate the functionality of the <pre><code> block features of the online editor here on SO.

After obtaining each block and before wrapping in a <pre><code>, all markup entities should be converted (ie; like < to <, etc). That being said, I didn’t do that step in the Ruby code sample below. I do have the regex’s to do that though.

A special note about trimming: The main regex below does not include residual trailing newlines. Nor does the SO functionality. So the code block is correct top to bottom.
However, the leading 4 spaces (or tab) that could be contained in the body can’t be trimmed (and they should be) in the main regex. For that it needs a callback.

Playing around with the gsub block mode, its easy to trim those leading spaces/tab.

Let me know if you have any problems with this.

Links –
Rubular (for the regex): http://rubular.com/r/pp9oRLQ0xo
Ideone (for the working Ruby code): http://ideone.com/aA9it

Regex compressed –
(^\s*$\n|\A)(^(?:[ ]{4}|\t).*[^\s].*$\n?(?:(?:^\s*$\n?)*^(?:[ ]{4}|\t).*[^\s].*$\n?)*)

Regex expanded –

(^\s*$\n|\A)                                # Capt grp 1, block is preceeded by a blank line or begin of string
(                                           # Begin "Capture group 2", start of pre/code block
   ^(?:[ ]{4}|\t) .* [^\s] .* $ \n?            # First line of code block (note - lines must contain at least 1 non-whitespace character)
   (?:                                         # Start "Optionally, get more lines of code"
       (?: ^ \s* $ \n? )*                         # Many optional blank lines
       ^(?:[ ]{4}|\t) .* [^\s] .* $ \n?           # Another line of code 
   )*                                          # End "Optionally, get more lines of code", do 0 or more times
)                                           # End "Capture group 2", end of pre/code block

Ruby code –

regex = /(^\s*$\n|\A)(^(?:[ ]{4}|\t).*[^\s].*$\n?(?:(?:^\s*$\n?)*^(?:[ ]{4}|\t).*[^\s].*$\n?)*)/;

data = '
Hello Worldsasdasdffasdfasdf  asdf

    thisdqweee

    asdfasdfasdfasdf
sdfg

    #YYYY {
    height: 100%;
    min-height: 800px;
    margin-right: 20px;
    position: relative;
    }


    #ZZZZZZ {
    height: 100%;
    overflow: hidden;
    }';


# ---
result = data.gsub(regex) {
   ||
   x=$2;
     ## Construct the return value '\1<pre&gt<code&gt\2</code&gt</pre&gt'.
     ## But, trim each line with 1 to 4 leading spaces (or a tab with regex on the bottom).
     ## They are not necessary now, they are replaced with a code block.

   $1 + '<pre&gt<code&gt' +   x.gsub(/^[ ]{1,4}/, '') + '</code&gt</pre&gt'
};

# Note - Tabs can be trimed too, use : x.gsub(/^(?:[ ]{1,4}|\t)/,'') in the above

print result;