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Asked: 2026-06-04T06:06:08+00:00

I am to find lines in a file whose fifth character is a digit.

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I am to find lines in a file whose fifth character is a digit. That is, of the digit set {0123456789}. I made a file that had:

asdf1
asdf2

.. and so on until 0.
I made a few more cases with special characters, and other symbols just to make sure I wasn’t grabbing those lines either.

However, one thing intrigued me. When making the line:

SSSSS3

or

TTTTTS7

that is, where every S is a space, and T is a tab, that line is also retrieved when the fifth character is a space/tab and not a number.

Could anybody explain why this is happening?
The pattern I’m using is 

'....[0-9](.)*'

If I’m not mistaken, a ‘.’ represents any character, right? How could more than 4 tabs or spaces count as less than 5 ‘.’?

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1 Answer

  1. Editorial Team

    You’re missing the start-of-line anchor:

    ^....[0-9]

    Without it, egrep performs a regexp search rather than a regexp match, as if you had entered

    .*....[0-9]

    though with the difference that the .* does not cause capturing for flags like -o.

    (Note that the (.)* at the end is also useless. .* is implicitly appended to the end of the RE unless you put in an end-of-line anchor, $.)

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