add chopprefix, chopsuffix

base/exports.jl

@@ -573,6 +573,8 @@ export
     bytes2hex,
     chomp,
     chop,
+    chopprefix,
+    chopsuffix,
     codepoint,
     codeunit,
     codeunits,

base/regex.jl

@@ -335,6 +335,20 @@ function endswith(s::SubString, r::Regex)
     return PCRE.exec_r(r.regex, s, 0, r.match_options | PCRE.ENDANCHORED)
 end
 
+function chopprefix(s::AbstractString, prefix::Regex)
+    m = match(prefix, s, firstindex(s), PCRE.ANCHORED)
+    m === nothing && return SubString(s)
+    return SubString(s, ncodeunits(m.match) + 1)
+end
+
+function chopsuffix(s::AbstractString, suffix::Regex)
+    m = match(suffix, s, firstindex(s), PCRE.ENDANCHORED)
+    m === nothing && return SubString(s)
+    isempty(m.match) && return SubString(s)
+    return SubString(s, firstindex(s), prevind(s, m.offset))
+end
+
+
 """
     match(r::Regex, s::AbstractString[, idx::Integer[, addopts]])
 

base/strings/util.jl

@@ -19,8 +19,13 @@ true
 ```
 """
 function startswith(a::AbstractString, b::AbstractString)
-    a, b = Iterators.Stateful(a), Iterators.Stateful(b)
-    all(splat(==), zip(a, b)) && isempty(b)
+    i, j = iterate(a), iterate(b)
+    while true
+        j === nothing && return true # ran out of prefix: success!
+        i === nothing && return false # ran out of source: failure
+        i[1] == j[1] || return false # mismatch: failure
+        i, j = iterate(a, i[2]), iterate(b, j[2])
+    end
 end
 startswith(str::AbstractString, chars::Chars) = !isempty(str) && first(str)::AbstractChar in chars
 
@@ -39,9 +44,14 @@ true
 ```
 """
 function endswith(a::AbstractString, b::AbstractString)
-    a = Iterators.Stateful(Iterators.reverse(a))
-    b = Iterators.Stateful(Iterators.reverse(b))
-    all(splat(==), zip(a, b)) && isempty(b)
+    a, b = Iterators.Reverse(a), Iterators.Reverse(b)
+    i, j = iterate(a), iterate(b)
+    while true
+        j === nothing && return true # ran out of suffix: success!
+        i === nothing && return false # ran out of source: failure
+        i[1] == j[1] || return false # mismatch: failure
+        i, j = iterate(a, i[2]), iterate(b, j[2])
+    end
 end
 endswith(str::AbstractString, chars::Chars) = !isempty(str) && last(str) in chars
 
@@ -51,7 +61,7 @@ function startswith(a::Union{String, SubString{String}},
     if ncodeunits(a) < cub
         false
     elseif _memcmp(a, b, sizeof(b)) == 0
-        nextind(a, cub) == cub + 1
+        nextind(a, cub) == cub + 1 # check that end of `b` doesn't match a partial character in `a`
     else
         false
     end
@@ -64,7 +74,7 @@ function endswith(a::Union{String, SubString{String}},
     if astart < 1
         false
     elseif GC.@preserve(a, _memcmp(pointer(a, astart), b, sizeof(b))) == 0
-        thisind(a, astart) == astart
+        thisind(a, astart) == astart # check that end of `b` doesn't match a partial character in `a`
     else
         false
     end
@@ -195,6 +205,91 @@ end
 # TODO: optimization for the default case based on
 # chop(s::AbstractString) = SubString(s, firstindex(s), prevind(s, lastindex(s)))
 
+"""
+    chopprefix(s::AbstractString, prefix::Union{AbstractString,Regex}) -> SubString
+
+Remove the prefix `prefix` from `s`. If `s` does not start with `prefix`, a string equal to `s` is returned.
+
+See also [`chopsuffix`](@ref).
+
+!!! compat "Julia 1.8"
+    This function is available as of Julia 1.8.
+
+# Examples
+```jldoctest
+julia> chopprefix("Hamburger", "Ham")
+"burger"
+
+julia> chopprefix("Hamburger", "hotdog")
+"Hamburger"
+```
+"""
+function chopprefix(s::AbstractString, prefix::AbstractString)
+    k = firstindex(s)
+    i, j = iterate(s), iterate(prefix)
+    while true
+        j === nothing && i === nothing && return SubString(s, 1, 0) # s == prefix: empty result
+        j === nothing && return @inbounds SubString(s, k) # ran out of prefix: success!
+        i === nothing && return SubString(s) # ran out of source: failure
+        i[1] == j[1] || return SubString(s) # mismatch: failure
+        k = i[2]
+        i, j = iterate(s, k), iterate(prefix, j[2])
+    end
+end
+
+function chopprefix(s::Union{String, SubString{String}},
+                    prefix::Union{String, SubString{String}})
+    if startswith(s, prefix)
+        SubString(s, 1 + ncodeunits(prefix))
+    else
+        SubString(s)
+    end
+end
+
+"""
+    chopsuffix(s::AbstractString, suffix::Union{AbstractString,Regex}) -> SubString
+
+Remove the suffix `suffix` from `s`. If `s` does not end with `suffix`, a string equal to `s` is returned.
+
+See also [`chopprefix`](@ref).
+
+!!! compat "Julia 1.8"
+    This function is available as of Julia 1.8.
+
+# Examples
+```jldoctest
+julia> chopsuffix("Hamburger", "er")
+"Hamburg"
+
+julia> chopsuffix("Hamburger", "hotdog")
+"Hamburger"
+```
+"""
+function chopsuffix(s::AbstractString, suffix::AbstractString)
+    a, b = Iterators.Reverse(s), Iterators.Reverse(suffix)
+    k = lastindex(s)
+    i, j = iterate(a), iterate(b)
+    while true
+        j === nothing && i === nothing && return SubString(s, 1, 0) # s == suffix: empty result
+        j === nothing && return @inbounds SubString(s, firstindex(s), k) # ran out of suffix: success!
+        i === nothing && return SubString(s) # ran out of source: failure
+        i[1] == j[1] || return SubString(s) # mismatch: failure
+        k = i[2]
+        i, j = iterate(a, k), iterate(b, j[2])
+    end
+end
+
+function chopsuffix(s::Union{String, SubString{String}},
+                    suffix::Union{String, SubString{String}})
+    if !isempty(suffix) && endswith(s, suffix)
+        astart = ncodeunits(s) - ncodeunits(suffix) + 1
+        @inbounds SubString(s, firstindex(s), prevind(s, astart))
+    else
+        SubString(s)
+    end
+end
+
+
 """
     chomp(s::AbstractString) -> SubString
 

doc/src/base/strings.md

@@ -66,6 +66,8 @@ Base.uppercasefirst
 Base.lowercasefirst
 Base.join
 Base.chop
+Base.chopprefix
+Base.chopsuffix
 Base.chomp
 Base.thisind
 Base.nextind

test/strings/util.jl

@@ -1,5 +1,7 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
+SubStr(s) = SubString("abc$(s)de", firstindex(s) + 3, lastindex(s) + 3)
+
 @testset "padding (lpad and rpad)" begin
     @test lpad("foo", 2) == "foo"
     @test rpad("foo", 2) == "foo"
@@ -486,35 +488,91 @@ end
 end
 
 @testset "chomp/chop" begin
-    @test chomp("foo\n") == "foo"
-    @test chomp("fo∀\n") == "fo∀"
-    @test chomp("foo\r\n") == "foo"
-    @test chomp("fo∀\r\n") == "fo∀"
-    @test chomp("fo∀") == "fo∀"
-    @test chop("") == ""
-    @test chop("fooε") == "foo"
-    @test chop("foεo") == "foε"
-    @test chop("∃∃∃∃") == "∃∃∃"
-    @test chop("∀ϵ∃Δ", head=0, tail=0) == "∀ϵ∃Δ"
-    @test chop("∀ϵ∃Δ", head=0, tail=1) == "∀ϵ∃"
-    @test chop("∀ϵ∃Δ", head=0, tail=2) == "∀ϵ"
-    @test chop("∀ϵ∃Δ", head=0, tail=3) == "∀"
-    @test chop("∀ϵ∃Δ", head=0, tail=4) == ""
-    @test chop("∀ϵ∃Δ", head=0, tail=5) == ""
-    @test chop("∀ϵ∃Δ", head=1, tail=0) == "ϵ∃Δ"
-    @test chop("∀ϵ∃Δ", head=2, tail=0) == "∃Δ"
-    @test chop("∀ϵ∃Δ", head=3, tail=0) == "Δ"
-    @test chop("∀ϵ∃Δ", head=4, tail=0) == ""
-    @test chop("∀ϵ∃Δ", head=5, tail=0) == ""
-    @test chop("∀ϵ∃Δ", head=1, tail=1) == "ϵ∃"
-    @test chop("∀ϵ∃Δ", head=2, tail=2) == ""
-    @test chop("∀ϵ∃Δ", head=3, tail=3) == ""
-    @test_throws ArgumentError chop("∀ϵ∃Δ", head=-3, tail=3)
-    @test_throws ArgumentError chop("∀ϵ∃Δ", head=3, tail=-3)
-    @test_throws ArgumentError chop("∀ϵ∃Δ", head=-3, tail=-3)
-
-    @test isa(chomp("foo"), SubString)
-    @test isa(chop("foo"), SubString)
+    for S in (String, SubStr, Test.GenericString)
+        @test chomp(S("foo\n")) == "foo"
+        @test chomp(S("fo∀\n")) == "fo∀"
+        @test chomp(S("foo\r\n")) == "foo"
+        @test chomp(S("fo∀\r\n")) == "fo∀"
+        @test chomp(S("fo∀")) == "fo∀"
+        @test chop(S("")) == ""
+        @test chop(S("fooε")) == "foo"
+        @test chop(S("foεo")) == "foε"
+        @test chop(S("∃∃∃∃")) == "∃∃∃"
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=0) == "∀ϵ∃Δ"
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=1) == "∀ϵ∃"
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=2) == "∀ϵ"
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=3) == "∀"
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=4) == ""
+        @test chop(S("∀ϵ∃Δ"), head=0, tail=5) == ""
+        @test chop(S("∀ϵ∃Δ"), head=1, tail=0) == "ϵ∃Δ"
+        @test chop(S("∀ϵ∃Δ"), head=2, tail=0) == "∃Δ"
+        @test chop(S("∀ϵ∃Δ"), head=3, tail=0) == "Δ"
+        @test chop(S("∀ϵ∃Δ"), head=4, tail=0) == ""
+        @test chop(S("∀ϵ∃Δ"), head=5, tail=0) == ""
+        @test chop(S("∀ϵ∃Δ"), head=1, tail=1) == "ϵ∃"
+        @test chop(S("∀ϵ∃Δ"), head=2, tail=2) == ""
+        @test chop(S("∀ϵ∃Δ"), head=3, tail=3) == ""
+        @test_throws ArgumentError chop(S("∀ϵ∃Δ"), head=-3, tail=3)
+        @test_throws ArgumentError chop(S("∀ϵ∃Δ"), head=3, tail=-3)
+        @test_throws ArgumentError chop(S("∀ϵ∃Δ"), head=-3, tail=-3)
+
+        for T in (String, SubStr, Test.GenericString, Regex)
+            S === Test.GenericString && T === Regex && continue # not supported
+            @test chopprefix(S("fo∀\n"), T("bog")) == "fo∀\n"
+            @test chopprefix(S("fo∀\n"), T("\n∀foΔ")) == "fo∀\n"
+            @test chopprefix(S("fo∀\n"), T("∀foΔ")) == "fo∀\n"
+            @test chopprefix(S("fo∀\n"), T("f")) == "o∀\n"
+            @test chopprefix(S("fo∀\n"), T("fo")) == "∀\n"
+            @test chopprefix(S("fo∀\n"), T("fo∀")) == "\n"
+            @test chopprefix(S("fo∀\n"), T("fo∀\n")) == ""
+            @test chopprefix(S("\nfo∀"), T("bog")) == "\nfo∀"
+            @test chopprefix(S("\nfo∀"), T("\n∀foΔ")) == "\nfo∀"
+            @test chopprefix(S("\nfo∀"), T("\nfo∀")) == ""
+            @test chopprefix(S("\nfo∀"), T("\n")) == "fo∀"
+            @test chopprefix(S("\nfo∀"), T("\nf")) == "o∀"
+            @test chopprefix(S("\nfo∀"), T("\nfo")) == "∀"
+            @test chopprefix(S("\nfo∀"), T("\nfo∀")) == ""
+            @test chopprefix(S(""), T("")) == ""
+            @test chopprefix(S(""), T("asdf")) == ""
+            @test chopprefix(S(""), T("∃∃∃")) == ""
+            @test chopprefix(S("εfoo"), T("ε")) == "foo"
+            @test chopprefix(S("ofoε"), T("o")) == "foε"
+            @test chopprefix(S("∃∃∃∃"), T("∃")) == "∃∃∃"
+            @test chopprefix(S("∃∃∃∃"), T("")) == "∃∃∃∃"
+
+            @test chopsuffix(S("fo∀\n"), T("bog")) == "fo∀\n"
+            @test chopsuffix(S("fo∀\n"), T("\n∀foΔ")) == "fo∀\n"
+            @test chopsuffix(S("fo∀\n"), T("∀foΔ")) == "fo∀\n"
+            @test chopsuffix(S("fo∀\n"), T("\n")) == "fo∀"
+            @test chopsuffix(S("fo∀\n"), T("∀\n")) == "fo"
+            @test chopsuffix(S("fo∀\n"), T("o∀\n")) == "f"
+            @test chopsuffix(S("fo∀\n"), T("fo∀\n")) == ""
+            @test chopsuffix(S("\nfo∀"), T("bog")) == "\nfo∀"
+            @test chopsuffix(S("\nfo∀"), T("\n∀foΔ")) == "\nfo∀"
+            @test chopsuffix(S("\nfo∀"), T("\nfo∀")) == ""
+            @test chopsuffix(S("\nfo∀"), T("∀")) == "\nfo"
+            @test chopsuffix(S("\nfo∀"), T("o∀")) == "\nf"
+            @test chopsuffix(S("\nfo∀"), T("fo∀")) == "\n"
+            @test chopsuffix(S("\nfo∀"), T("\nfo∀")) == ""
+            @test chopsuffix(S(""), T("")) == ""
+            @test chopsuffix(S(""), T("asdf")) == ""
+            @test chopsuffix(S(""), T("∃∃∃")) == ""
+            @test chopsuffix(S("fooε"), T("ε")) == "foo"
+            @test chopsuffix(S("εofo"), T("o")) == "εof"
+            @test chopsuffix(S("∃∃∃∃"), T("∃")) == "∃∃∃"
+            @test chopsuffix(S("∃∃∃∃"), T("")) == "∃∃∃∃"
+        end
+        @test isa(chomp(S("foo")), SubString)
+        @test isa(chop(S("foo")), SubString)
+
+        if S !== Test.GenericString
+            @test chopprefix(S("∃∃∃b∃"), r"∃+") == "b∃"
+            @test chopsuffix(S("∃b∃∃∃"), r"∃+") == "∃b"
+        end
+
+        @test isa(chopprefix(S("foo"), "fo"), SubString)
+        @test isa(chopsuffix(S("foo"), "oo"), SubString)
+    end
 end
 
 @testset "bytes2hex and hex2bytes" begin