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deleted 3 characters in body
Kusalananda
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  • 970

The pattern [a-z]* matches zero or more characters in the range a to z. There are zero such characters at the very start of the string 123 abc (i.e. the pattern matches), and also four of them at the start of this is a line.

If you need at least one match, then use [a-z][a-z]* or enable extended regular expressions with sed -E and use [a-z]+.

Kusalananda
  • 332.9k
  • 37
  • 673
  • 970