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Regular expression for words containing letters in alphabetical order JAVA

My student (high school) asked me a question about a regular expression corresponding to a word containing letters in alphabetical order. Honestly, I do not know how to create a regular expression. An example of matching words, the size of the letters does not matter:

abc, aWZ, gOR, bor
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 ^a*b*c*d*e*f*g*h*i*j*k*l*m*n*o*p*q*r*s*t*u*v*w*x*y*z*$

must work

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This should work:

 (?i)a*b*c*...z*

It would be easy to build in a loop.

 StringBuilder b = new StringBuilder(64); b.append("(?i)"); for (int i = 'a'; i <= 'z'; i++) b.append((char)i).append('*'); return Pattern.compile(b.toString()).matcher(input).matches();
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This is not a good problem for regex, it is almost certainly easier to check it in a for loop:

 import static java.lang.Character.isLetter; import static java.lang.Character.toLowerCase; public boolean alphabeticalOrder(String word) { for (int i = 0; i < word.length() - 1; i++) { if (!isLetter(word.charAt(i)) || toLowerCase(word.charAt(i + 1)) < toLowerCase(word.charAt(i))) return false; } return isLetter(word.charAt(word.length() - 1)); }

Alternatively, the following regular expression will also match correctly (only for ASCII alphabetic characters) when passed to CASE_INSENSITIVE :

 ^a*b*c*d*e*f*g*h*i*j*k*l*m*n*o*p*q*r*s*t*u*v*w*x*y*z*$
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