Regex (ECMAScript), 19 bytes

/\bill(?![aiouy])/i

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Regular expression

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiouy]) // not immediately followed by a, i, o, u or y
/i            // in case-insensitive mode

Regex (ECMAScript), 19 bytes

/\bill(?![aiouy])/i

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Explanation

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiouy]) // not immediately followed by a, i, o, u or y
/i            // in case-insensitive mode

JavaScript (ES6), 29 bytes

(+1 byte if y counts as a vowel)

s=>/\bill(?![aiou])/i.test(s)

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Regular expression

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiou])  // not immediately followed by a, i, o or u
/i            // in case-insensitive mode

Regex (ECMAScript), 19 bytes

/\bill(?![aiouy])/i

Try it online!

Regular expression

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiouy]) // not immediately followed by a, i, o, u or y
/i            // in case-insensitive mode

JavaScript (ES6), 29 bytes

(+1 byte if y counts as a vowel)

s=>/\bill(?![aiou])/i.test(s)

Try it online!

Regular expression

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiou])  // not followed by a, i, o or u
/i            // in case-insensitive mode

JavaScript (ES6), 29 bytes

(+1 byte if y counts as a vowel)

s=>/\bill(?![aiou])/i.test(s)

Try it online!

Regular expression

We look for ...

/             //
  \b          // a word boundary
  ill         // followed by "ill"
  (?![aiou])  // not immediately followed by a, i, o or u
/i            // in case-insensitive mode