I'm trying to use a regular expression in the man page of Bash by using less.

I press / in less to enter a pattern, and I type z and press the Enter. I expected it to not match upper-case z (Z), but it does.

How do I make it not match Z? What kind of regular expressions are these that are not case sensitive?


2 Answers 2


Pretty sure you can get around that by using -i or +i in order to set less to default.


It's explained in the man page for less.

The default action for REs is to ignore case if there are no uppercase characters present, but to act case-sensitively otherwise.

There are three modes available within less:

  1. Case context dependent: a search or RE without uppercase characters is considered to be case-insensitive, but a search or RE containing at least one uppercase character is considered to be case-sensitive. Examples: abc will match abc and aBC, but aBc will only match aBc and not abc or ABC. This is the default setting.
  2. Case sensitive: a search or RE pays full regard to the case of any letter. Example: abC will match only abC and not abc or ABC.
  3. Case insensitive: a search or RE pays no regard to the case of any letter. Example: abC will match any of abc, abC, or ABC.

You can toggle case sensitive comparisons with -I, and case context sensitive comparisons with -i.

The control can be specified in three ways:

  • On the command line, for example less -I bigfile.txt.
  • In the environment, for example export LESS=-i and later less bigfile.txt.
  • Within less itself, for example by starting less bigfile.txt and then typing -i.
  • Hey, roaima. I think this is incorrect. ``` -i or --ignore-case Causes searches to ignore case; that is, uppercase and lowercase are considered identical. This option is ignored if any uppercase letters appear in the search pattern; in other words, if a pattern contains uppercase letters, then that search does not ignore case. -I or --IGNORE-CASE Like -i, but searches ignore case even if the pattern contains uppercase letters. ``` From my experiments, it behaves as -I, not as -i.
    – regex
    Apr 10, 2019 at 23:14
  • 1
    You could also append |$A (which translates as or A after the end of the line) for your pattern to become case sensitive without having to the change the settings. Apr 11, 2019 at 16:35
  • @StéphaneChazelas that's thrown me. I had thought $ was only special at the end of an RE (or RE clause), so $A would match a dollar and a capital letter, but A$ would match a capital at the end of a line. (Update: I can't even mimic this with perl, but definitely works with less.) Apr 11, 2019 at 16:50
  • 1
    @roaima, it's different between BRE and ERE, POSIX requires BRE $a to match on $a, and ERE $a to not match ($ to match the end of the subject wherever it's found). In Perl/PCRE, $ matches at the end of the subject or before a trailing line delimiter at the end of the subject, or if the m flag is enabled ((?m)) at the end of the subject or before any line delimiter in the subject. Apr 11, 2019 at 17:26

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