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Asked: 2026-06-06T20:58:19+00:00

I would like to understand the difference between the following 3 regular expressions: I

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I would like to understand the difference between the following 3 regular expressions:

I wanted to display all the lines in a file that consisted only of lowercase alphabets in them.

Here are the 3 regular expressions I tried:

cat filename.txt | grep ^[a-z]*

Regex Description: This will display all the lines starting with 0 or more lowercase letters. So, it will match either of the following:

zapato
113078
OLIVIA

Not exactly, what we wanted.

cat filename.txt | grep ^[a-z]*$

Regex Description: This will display all the lines starting with 0 or more lowercase letters till the end of the line. This matches the following:

fubuki
BALLIN
Kristine

This time there were no results with digits in them.

cat filename.txt | grep ^[a-z]*[a-z]$

Regex Description: This one works well for me. It searches for all the lines starting with 0 or more lowercase letters and it matches it till it finds another lowercase letter. For some reason, this works for me. However, I want to know how this is different from the previous regular expressions.

tonia
ecurby
totonno

Also, when the asterisk () in the regular expression means, 0 or more, then it should include all the results when I write, ^[a-z]

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1 Answer

  1. Editorial Team

    Short explanations of your regular expressions:

    ^[a-z]*

    Match string starting with 0 or more characters from [a-z].
    Matches empty string and every string starting with character of set [a-z].

    ^[a-z]*$

    Match string containing nothing but 0 or more characters from [a-z].
    Matches empty string and every string containing only characters of set [a-z].

    ^[a-z]*[a-z]$

    Match string starting with 0 or more characters from [a-z] followed by exactly one last character from [a-z].
    Matches every non-empty string containing only characters of set [a-z].

    Use this instead of your current third option:

    ^[a-z]+$

    It is semantically equivalent but simpler.

    The expression x*x (or xx*) is equivalent to x+ in regular expressions (with x being any expression). The latter is basically just syntactic sugar for either of the former more verbose expressions.

    Or put differently: while * means 0 or more, + means 1 or more.

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