Color-code a new generic character combination

Question

I'm new to Emacs and Org-mode, this is a very basic question. Unfortunately the keywords are too common for me to find the answer.

I would like to create a custom pattern to colour my writing input.

Something like

_theorem: (it would detect _ and a non-whitespace character for the start, and would trace it until the end, or colon)

_theorem 2: (it would be great if this could also include whitespaces)

_theorem on topic 1: (such as this).

The script would thus automatically color all of the above writing that is now in bold. References that would help me do this would also be helpful. Many thanks for helping a beginner!

Update

I've added a follow-up question in the comments on adding multiple such rules that ran into problems. Here's the full code for it. While the first answer to the first question is working perfectly now, the other two new rules seem to be mistakenly formulated or in conflict somehow. On the instances where either of them is found, it breaks the regular coloring of org-mode and does no coloring itself.

(add-hook 'org-mode-hook
          (lambda ()
                (font-lock-add-keywords nil
                                    '((
                                    "\\(^\\|\\s-+\\)\\(_\\([a-zA-Z0-9]+\\s-?\\)\\{0,4\\}:\\)" 2
                                    font-lock-type-face t)))
                (font-lock-add-keywords nil
                                    '((
                                    "\\(\\w+\\(\\s-&\\s-\\w+\\)?,\\s-[0-9]\\{4\\})" 2
                                    font-lock-type-face t)))
                (font-lock-add-keywords nil
                                    '((
                                    "^\\w+:" 2
                                       font-lock-type-face t)))
))

Answer 1

You can use font-lock-add-keywords to add custom highlights.

In the below example, I am highlighting any string

• that begins with _ preceded by a white-space character or beginning of the line
• followed by a single alpha-numeric character
• followed by anything else
• ending with :

The highlight color chosen is font-lock-warning-face; you can choose any other face of your liking.

This highlighting will happen only in org-mode.

(add-hook 'org-mode-hook
          (lambda ()
            (font-lock-add-keywords nil
                                    '(("\\(^\\|\\s-+\\)\\(_[a-zA-Z0-9].*?:\\)" 2
                                       font-lock-warning-face t)))))

Update:

To match at most 4 words before the colon,

(add-hook 'org-mode-hook
          (lambda ()
            (font-lock-add-keywords nil
                                    '(("\\(^\\|\\s-+\\)\\(_\\w+\\(?:\\s-+\\w+\\)\\{0,3\\}\\s-*:\\)" 2
                                       font-lock-warning-face t)))))

Update 2:

Preview of the "max 4 words" regexp in re-builder:

Update 3:

Alright, I can't keep on updating this as your question changes. I would suggest that you spend adequate time understanding the elisp regular expression syntax and how the font-lock-add-keywords function works.

• Elisp Regular Expression
• font-lock-add-keywords - Do C-h f font-lock-add-keywords RET in emacs and then click on the font-lock.el hyperlink in the buffer that opens. That will take you to the function definition. Read the doc-string for that function.
• In a similar fashion, you can learn more about any variable by doing C-h v VARIABLE-NAME RET and jumping through the hyperlinks.
• Keep on experimenting with re-builder till you get a hang of the regexp syntax.

Below is the solution to your last modified request.

(add-hook 'org-mode-hook
          (lambda ()
            (font-lock-add-keywords
             nil
             '(
               ;; Highlight upto 4 words (including ,) between ( and )
               ("\\(([a-zA-Z0-9,]+\\(?:\\s-+[a-zA-Z0-9,]+\\)\\{0,3\\})\\)" 1 font-lock-type-face)
               ;; Highlight ^word:
               ("^\\([a-zA-Z0-9]+:\\)" 1 font-lock-type-face)
               ;; Highlight upto 4 words between _ and :
               ("\\(^\\|\\s-+\\)\\(_[a-zA-Z0-9]+\\(?:\\s-+[a-zA-Z0-9]+\\)\\{0,3\\}\\s-*:\\)" 2 font-lock-type-face)
               ))))

Alternative Solution, does the same thing as in Update 3

This approach updates font-lock-keywords-alist instead of polluting font-lock-keywords. There is no easy way to fix incorrect regexp added to font-lock-keywords other than restarting emacs.

But usually font-lock-keywords-alist is much smaller in size and can be tweaked manually or can be reset completely by doing

(setq font-lock-keywords-alist nil)

while you are testing out the regexp.

(font-lock-add-keywords
 'org-mode
 '(
   ;; Highlight upto 4 words (including ,) between ( and )
   ("\\(([a-zA-Z0-9,]+\\(?:\\s-+[a-zA-Z0-9,]+\\)\\{0,3\\})\\)" 1 font-lock-type-face)
   ;; Highlight ^word:
   ("^\\([a-zA-Z0-9]+:\\)" 1 font-lock-type-face)
   ;; Highlight upto 4 words between _ and :
   ("\\(^\\|\\s-+\\)\\(_[a-zA-Z0-9]+\\(?:\\s-+[a-zA-Z0-9]+\\)\\{0,3\\}\\s-*:\\)" 2 font-lock-type-face)
   )
 )

Answer 2

A while ago, I posted a similar question here: check How to highlight text permanently in org-mode

Not exactly what you are asking for, this gives you more, customize it as you like:

(setq org-emphasis-alist
      '(
        ("*" (:foreground "cyan" :weight bold))
        ("/" (:foreground "cyan" :slant italic))
        ("_" (:foreground "cyan" :underline t))
        ("=" (:foreground "cyan" :weight bold))
        ("~" (:foreground "cyan" :weight bold
                          :box (:line-width 3 :color "#545454" :style released-button)))
        ("+" (:foreground "cyan" :strike-through t))))