Feature #12694: Want a String method to remove heading substr

• Status: Closed
• Assignee: -
• Target version: -

[sonots (Naotoshi Seo)]

I often write codes like:

str = 'abcdef'
substr = 'abc'
str[substr.size..-1] if str.start_with?(substr) #=> 'def'
# or
str.sub(/\A#{Regexp.escape(substr)}/, '') #=> 'def'

I want a short hand which is something like:

str = 'abcdef'
substr = 'abc'
str.lstrip(substr) #=> 'def'

Having similar argument for String#rstrip would be nice for symmetry although we already have String#chomp(substr)

My frequent use-case is coming from fluentd. We have remove_tag_prefix option to remove a heading substring from a tag. We currently using String#sub with a regular expression, but I feel regular expression matching is too much just to remove a heading substring. https://github.com/fluent/fluentd/blob/75005f18d48ed3d416890413fa5f83982b264c71/lib/fluent/compat/handle_tag_name_mixin.rb#L45

[nobu (Nobuyoshi Nakada)]

IIRC, there were proposals of lstrip and rstrip enhancement.
I'd expect them to accept char class(es), than a sub-string.

Note that rb_str_index_m accepts a Regexp too.
Calling str_strlen on it will segfault.

[sonots (Naotoshi Seo)]

Thanks, I found an issue which is 3 years old https://bugs.ruby-lang.org/issues/6842

[shyouhei (Shyouhei Urabe)]

• Is duplicate of Feature #6842: Add Optional Arguments to String#strip added

[sonots (Naotoshi Seo)]

• Status changed from Open to Rejected

I will continue this issue on https://bugs.ruby-lang.org/issues/6842, let me close.

=> https://bugs.ruby-lang.org/issues/6842 is too broad. Let us reopen this ticket, and talk about a way removing heading substr at this thread.

[sonots (Naotoshi Seo)]

• Status changed from Rejected to Open

[mrkn (Kenta Murata)]

This method removes prefix string, so I propose remove_prefix or deprefix (means remove prefix).

[sonots (Naotoshi Seo)]

We discussed about this ticket on ruby development meeting.

Python lstrip http://www.tutorialspoint.com/python/string_lstrip.htm takes character classes as an argument, so providing a substring as an argument will introduce confusion.

We need to think of new naming. What about String#lchomp or String#remove_prefix or String#head_chop for example?

[sonots (Naotoshi Seo)]

It seems python, perl does not have similar methods (which removes heading substr)

[sonots (Naotoshi Seo)]

• Description updated (diff)

[sonots (Naotoshi Seo)]

• Description updated (diff)

[sonots (Naotoshi Seo)]

• Description updated (diff)

[sonots (Naotoshi Seo)]

PHP has ltrim and rtrim (which is an alias of chop), but they are for removing a character list, not removing a substring http://php.net/manual/en/function.ltrim.php

ltrim('foaofe', 'foa'); #=> 'e'

Elixir has trim_leading, but it looks different with what we want to do http://elixir-lang.org/docs/stable/elixir/String.html#trim_leading/2

iex> String.trim_leading("__ abc _", "_")
" abc _"

iex> String.trim_leading("1 abc", "11")
"1 abc"

Golang has the same one with what we want to do https://golang.org/pkg/strings/#TrimPrefix and TrimSuffix

	var s = "Goodbye,, world!"
	s = strings.TrimPrefix(s, "Goodbye,") #=> , world

[duerst (Martin Dürst)]

Naotoshi Seo wrote:

Python lstrip http://www.tutorialspoint.com/python/string_lstrip.htm takes character classes as an argument, so providing a substring as an argument will introduce confusion.

"character classes" may be confusing. "a list of characters" might be more precise. There are many classes of characters that can't be expressed easily with a list. In particular because Python lstrip doesn't accept hyphens to group characters.

One way to deal with this would be to allow a Regexp that selects a single character, and apply this repeatedly. But we don't have any such concept yet.

[sonots (Naotoshi Seo)]

I've created a PR https://github.com/ruby/ruby/pull/1632 with a name String#remove_prefix.
I will merge if the name is acceptable.

[znz (Kazuhiro NISHIYAMA)]

str.sub(/^#{Regexp.escape(substr)}/, '') #=> 'def'

I think ^ should be \A in description of this ticket. (and maybe in handle_tag_name_mixin.rb of fluentd too)

Which is expected?: first prefix of string only or prefix of each line

[sonots (Naotoshi Seo)]

\A is correct. I updated the description, thanks.

[sonots (Naotoshi Seo)]

• Description updated (diff)

[matz (Yukihiro Matsumoto)]

I'd pick the name delete_prefix.

Matz.

[sonots (Naotoshi Seo)]

Thanks! I've changed the name to delete_prefix. https://github.com/ruby/ruby/pull/1632

[Hanmac (Hans Mackowiak)]

sonots (Naotoshi Seo) wrote:

Thanks! I've changed the name to delete_prefix. https://github.com/ruby/ruby/pull/1632

do we need a delete_suffix too or is that done via another method?

[sonots (Naotoshi Seo)]

We have String#chomp to delete suffix, but it would be nice to have the method for symmetry.
I will create another thread to discuss about it.

[sonots (Naotoshi Seo)]

I made a ticket for String#delete_suffix https://bugs.ruby-lang.org/issues/13665

[sonots (Naotoshi Seo)]

• Status changed from Open to Closed

Applied in changeset trunk|r59132.

---

string.c: add String#delete_prefix and String#delete_prefix!
to remove leading substr [Feature #12694] [fix GH-1632]

• string.c (rb_str_delete_prefix_bang): add a new method
  to remove prefix destuctively.

• string.c (rb_str_delete_prefix): add a new method
  to remove prefix non-destuctively.

• test/ruby/test_string.rb: add tests.