正则表达式：查找由某些字符组成的单词

Question

I have a list of dictionary words, I would like to find any word that consists of (some or all) certain characters of a source word in any order :

For Example:

Characters (source word) to look for : stainless

Found Words : stainless, stain, net, ten, less, sail, sale, tale, tales, ants, etc.

Also if a letter is found once in the source word it can't be repeated in the found word

Unacceptable words to find : tent (t is repeated), tall (l is repeated) , etc.

Acceptable words to find : less (s is already repeated in the source word), etc.

Answer 1

You could take this approach:

• Match any sequence of characters that are in the search word, requiring that the match is a word (word-boundaries)
• Prohibit that a certain character occurs more often than it is present in the search word, using a negative look-ahead. Do this for every character that is in the search word.

For the given example the regular expression would be:

(?!(\S*s){4}|(\S*t){2}|(\S*a){2}|(\S*i){2}|(\S*n){2}|(\S*l){2}|(\S*e){2})\b[stainless]+\b

The biggest part of the pattern deals with the negative look-ahead. For example:

• (\S*s){4} would match four times an 's' in a single word.
• (?! | ) places these patterns as different options in a negative look-ahead so that none of them should match.

Automation

It is clear that making such a regular expression for a given word needs some work, so that is where you could use some automation. Notepad++ cannot help with that, but in a programming environment it is possible. Here is a little snippet in JavaScript that will give you the regular expression that corresponds to a given search word:

 function regClassEscape(s) { // Escape "[" and "^" and "-": return s.replace(/[\]^-]/g, "\\$&"); } function buildRegex(searchWord) { // get frequency of each letter: let freq = {}; for (let ch of searchWord) { ch = regClassEscape(ch); freq[ch] = (freq[ch] ?? 0) + 1; } // Produce negative options (too many occurrences) const forbidden = Object.entries(freq).map(([ch, count]) => "(\\S*[" + ch + "]){" + (count + 1) + "}" ).join("|"); // Produce character set const allowed = Object.keys(freq).join(""); return "(?!" + forbidden + ")\\b[" + allowed + "]+\\b"; } // I/O management const [input, output] = document.querySelectorAll("input,div"); input.addEventListener("input", refresh); function refresh() { if (/\s/.test(input.value)) { output.textContent = "Input should have no white space!"; } else { output.textContent = buildRegex(input.value); } } refresh();

 input { width: 100% }

 Search word:<br> <input value="stainless"> Regular expression: <div></div>