add strutils.removePrefix proc (#6473)

lib/pure/strutils.nim

@@ -2345,8 +2345,7 @@ proc removeSuffix*(s: var string, chars: set[char] = Newlines) {.
 
 proc removeSuffix*(s: var string, c: char) {.
   rtl, extern: "nsuRemoveSuffixChar".} =
-  ## Removes a single character (in-place) from a string.
-  ##
+  ## Removes a single character (in-place) from the end of a string.
   ## .. code-block:: nim
   ##   var
   ##     table = "users"
@@ -2368,6 +2367,43 @@ proc removeSuffix*(s: var string, suffix: string) {.
     newLen -= len(suffix)
     s.setLen(newLen)
 
+proc removePrefix*(s: var string, chars: set[char] = Newlines) {.
+  rtl, extern: "nsuRemovePrefixCharSet".} =
+  ## Removes all characters from `chars` from the start of the string `s`
+  ## (in-place).
+  ## .. code-block:: nim
+  ##   var userInput = "\r\n*~Hello World!"
+  ##   userInput.removePrefix
+  ##   doAssert userInput == "*~Hello World!"
+  ##   userInput.removePrefix({'~', '*'})
+  ##   doAssert userInput == "Hello World!"
+  ##
+  ##   var otherInput = "?!?Hello!?!"
+  ##   otherInput.removePrefix({'!', '?'})
+  ##   doAssert otherInput == "Hello!?!"
+  var start = 0
+  while start < s.len and s[start] in chars: start += 1
+  if start > 0: s.delete(0, start - 1)
+
+proc removePrefix*(s: var string, c: char) {.
+  rtl, extern: "nsuRemovePrefixChar".} =
+  ## Removes a single character (in-place) from the start of a string.
+  ## .. code-block:: nim
+  ##   var ident = "pControl"
+  ##   ident.removePrefix('p')
+  ##   doAssert ident == "Control"
+  removePrefix(s, chars = {c})
+
+proc removePrefix*(s: var string, prefix: string) {.
+  rtl, extern: "nsuRemovePrefixString".} =
+  ## Remove the first matching prefix (in-place) from a string.
+  ## .. code-block:: nim
+  ##   var answers = "yesyes"
+  ##   answers.removePrefix("yes")
+  ##   doAssert answers == "yes"
+  if s.startsWith(prefix):
+    s.delete(0, prefix.len - 1)
+
 when isMainModule:
   doAssert align("abc", 4) == " abc"
   doAssert align("a", 0) == "a"