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Where can I find reference for less regex search patterns?

I want to search file with less using \d to find digits, but it does not seem to understand this wildcard. I tried to find a reference for less regex patterns, but could not find anything, not on man pages and not on the Internet.

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2 Answers 2

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less's man page says:

   /pattern
          Search forward in the file for the N-th line containing
          the pattern.  N defaults to 1.  The pattern is a regular
          expression, as recognized by the regular expression library
          supplied by your system.

so the accepted syntax may depend on your system. Off-hand, it seems to accept extended regular expressions on my Debian system, see regex(7), and Why does my regular expression work in X but not in Y?

\d is from Perl, and isn't supported by all regex engines. Use [0-9] or [[:digit:]] to match digits. (Their exact behaviour may depend on the locale.)

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    > as recognized by the regular expression library supplied by your system. < so… any way to direct less to libpcre? Commented Jan 28, 2021 at 20:54
  • I have Debian 10 according to lsb_release. If I run less on a file that contains a's and e's, and use the command /a|e/, less only highlights the a's. If I enter the command /a\|e/ I get pattern not found. This tends to support that less is accepting the basic syntax, according to man re_format. If I made a mistake in trying to invoke extended syntax, please let me know. Commented Jun 9, 2022 at 22:51
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    @cardiffspaceman, did you put the trailing / there? I don't think less expects that, so it'll look for the string e/ as the other alternative. It has to be extended regexes, since BRE doesn't have the alternation, and in BRE, a|e would look for that literal string
    – ilkkachu
    Commented Jun 9, 2022 at 23:02
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    @likkachu leaving off the trailing / does make the command work as expected, extended RE. Commented Jun 15, 2022 at 1:09
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The expressions supported by less are documented in the re_format(7) manual (man 7 re_format). That manual describes both the extended regular expressions and the basic regular expressions available on your system. The less utility understands extended regular expressions.

To match a digit, you would use [0-9] or [[:digit:]] (there's a slight difference as the former depends on the current locale). The \d pattern is a Perl-like regular expression (PCRE), not supported by less.

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