String Recipes

wiki

This is intended to be just short snippets, not fully developed functions which would belong in a library. Please feel free to add your favourites.

Changing case

Change the first character of a word to upper case

str = str:gsub("^%l", string.upper)

Change the first alphabetic character of a word to upper case

str = str:gsub("%a", string.upper, 1)

Put HTML tags in lowercase (but leaves attribute names untouched)

str = str:gsub("<[^%s>]+", string.lower)

Change an entire string to Title Case (i.e. capitalise the first letter of each word)

local function tchelper(first, rest)
  return first:upper()..rest:lower()
end
-- Add extra characters to the pattern if you need to. _ and ' are found in the
-- middle of identifiers and English words.
-- We must also put %w_' into [%w_'] to make it handle normal stuff and extra stuff the same.
-- This also turns hex numbers into, eg. 0Xa7d4
str = str:gsub("(%a)([%w_']*)", tchelper)

Example:

> str = "foo"
> str = str:gsub("^%l", string.upper)
> =str
Foo
> str = "_foo"
> str = str:gsub("^%l", string.upper)
> =str
_foo
> str = str:gsub("%a", string.upper, 1)
> =str
_Foo

Splitting strings

See also SplitJoin, MakingLuaLikePhp (explode).

Iterate over words in a string (adapted from Lua manual)

-- words and numbers
for word in str:gmatch("%w+") do ... end

-- identifiers in typical programming languages
for id in str:gmatch("[_%a][_%w]*") do ... end

-- whitespace-separated components (without handling quotes)
for id in str:gmatch("%S+") do ... end

Iterate over lines in a buffer, ignoring empty lines

for line in str:gmatch("[^\r\n]+") do ... end

Any of the above can also be done as a function iterator:

-- call func with each word in a string
str:gsub("%w+", func)

Break a string up at occurrences of a single character

If the number of fields is known:

str:match( ("([^"..sep.."]*)"..sep):rep(nsep) )

If the number of fields is not known

fields = {str:match((str:gsub("[^"..sep.."]*"..sep, "([^"..sep.."]*)"..sep)))}

Some might call the above a hack :) sep will need to be escaped if it is a pattern metacharacter, and you'd probably be better off precomputing and/or memoizing the patterns.

Yet another method:

fields = {}
str:gsub("([^"..sep.."]*)"..sep, function(c) table.insert(fields, c) end)

Wrap a string at a given margin

This is intended for strings without newlines in them (i.e. after reflowing the text and breaking it into paragraphs.)

function wrap(str, limit, indent, indent1)
  indent = indent or ""
  indent1 = indent1 or indent
  limit = limit or 72
  local here = 1-#indent1
  return indent1..str:gsub("(%s+)()(%S+)()",
                          function(sp, st, word, fi)
                            if fi-here > limit then
                              here = st - #indent
                              return "\n"..indent..word
                            end
                          end)
end

Reflowing text into paragraphs

This builds on wrap to do a quick-and-dirty reflow: paragraphs are defined as lines starting with a space, or having a blank line between them:

function reflow(str, limit, indent, indent1)
  return (str:gsub("%s*\n%s+", "\n")
             :gsub("%s%s+", " ")
             :gsub("[^\n]+",
                   function(line)
                     return wrap(line, limit, indent, indent1)
                   end))
end

Check if a string is a repetition of some pattern

"" == str:gsub(pat, "")

Check if a string is a repetition of some pattern separated by whitespace

not str:gsub(pat, ""):find"%S"

Example: split a string into words, or return nil

function justWords(str)
  local t = {}
  local function helper(word) table.insert(t, word) return "" end
  if not str:gsub("%w+", helper):find"%S" then return t end
end

Check if a string is a repetition of some pattern and also satisfies some condition

"" == str:gsub(pat, function(s) return ok(s) and "" or "*" end)

Split a string with a pattern, Take One

func((str:gsub("(.-)("..sep..")", func)))

Example: Split a string into lines separated by either DOS or Unix line endings, creating a table out of the results.

function lines(str)
  local t = {}
  local function helper(line) table.insert(t, line) return "" end
  helper((str:gsub("(.-)\r?\n", helper)))
  return t
end

Split a string with a pattern, Take Two

-- Compatibility: Lua-5.1
function split(str, pat)
   local t = {}  -- NOTE: use {n = 0} in Lua-5.0
   local fpat = "(.-)" .. pat
   local last_end = 1
   local s, e, cap = str:find(fpat, 1)
   while s do
      if s ~= 1 or cap ~= "" then
	 table.insert(t,cap)
      end
      last_end = e+1
      s, e, cap = str:find(fpat, last_end)
   end
   if last_end <= #str then
      cap = str:sub(last_end)
      table.insert(t, cap)
   end
   return t
end

Example: Split a file path string into components.

function split_path(str)
   return split(str,'[\\/]+')
end

parts = split_path("/usr/local/bin")
--> {'usr','local','bin'}

Test Cases:

split('foo/bar/baz/test','/')
--> {'foo','bar','baz','test'}
split('/foo/bar/baz/test','/')
--> {'foo','bar','baz','test'}
split('/foo/bar/baz/test/','/')
--> {'foo','bar','baz','test'}
split('/foo/bar//baz/test///','/')
--> {'foo','bar','','baz','test','',''}
split('//foo////bar/baz///test///','/+')
--> {'foo','bar','baz','test'}
split('foo','/+')
--> {'foo'}
split('','/+')
--> {}
split('foo','')  -- opps! infinite loop!

Split a string with a pattern, Take Three

-- Compatibility: Lua-5.0
function Split(str, delim, maxNb)
    -- Eliminate bad cases...
    if string.find(str, delim) == nil then
        return { str }
    end
    if maxNb == nil or maxNb < 1 then
        maxNb = 0    -- No limit
    end
    local result = {}
    local pat = "(.-)" .. delim .. "()"
    local nb = 0
    local lastPos
    for part, pos in string.gfind(str, pat) do
        nb = nb + 1
        result[nb] = part
        lastPos = pos
        if nb == maxNb then break end
    end
    -- Handle the last field
    if nb ~= maxNb then
        result[nb + 1] = string.sub(str, lastPos)
    end
    return result
end

Test Cases:

ShowSplit("abc", '')
--> { [1] = "", [2] = "", [3] = "", [4] = "", [5] = "" }
-- No infite loop... but garbage in, garbage out...
ShowSplit("", ',')
--> { [1] = "" }
ShowSplit("abc", ',')
--> { [1] = "abc" }
ShowSplit("a,b,c", ',')
--> { [1] = "a", [2] = "b", [3] = "c" }
ShowSplit("a,b,c,", ',')
--> { [1] = "a", [2] = "b", [3] = "c", [4] = "" }
ShowSplit(",a,b,c,", ',')
--> { [1] = "", [2] = "a", [3] = "b", [4] = "c", [5] = "" }
ShowSplit("x,,,y", ',')
--> { [1] = "x", [2] = "", [3] = "", [4] = "y" }
ShowSplit(",,,", ',')
--> { [1] = "", [2] = "", [3] = "", [4] = "" }
ShowSplit("x!yy!zzz!@", '!', 4)
--> { [1] = "x", [2] = "yy", [3] = "zzz", [4] = "@" }
ShowSplit("x!yy!zzz!@", '!', 3)
--> { [1] = "x", [2] = "yy", [3] = "zzz" }
ShowSplit("x!yy!zzz!@", '!', 1)
--> { [1] = "x" }

ShowSplit("a:b:i:p:u:random:garbage", ":", 5)
--> { [1] = "a", [2] = "b", [3] = "i", [4] = "p", [5] = "u" }
ShowSplit("hr , br ;  p ,span, div", '%s*[;,]%s*')
--> { [1] = "hr", [2] = "br", [3] = "p", [4] = "span", [5] = "div" }

(PhilippeLhoste)

Check if string X starts or ends with string Y

function string.starts(String,Start)
   return string.sub(String,1,string.len(Start))==Start
end

function string.ends(String,End)
   return End=='' or string.sub(String,-string.len(End))==End
end

Decode an URL-encoded string

(Note that you should only decode a URL string after splitting it; this allows you to correctly process quoted "?" characters in the query string or base part, for instance.)

function url_decode(str)
  str = string.gsub (str, "+", " ")
  str = string.gsub (str, "%%(%x%x)",
      function(h) return string.char(tonumber(h,16)) end)
  str = string.gsub (str, "\r\n", "\n")
  return str
end

}

URL-encode a string

function url_encode(str)
  if (str)
    str = string.gsub (str, "\n", "\r\n")
    str = string.gsub (str, "([^%w ])",
        function (c) return string.format ("%%%02X", string.byte(c)) end)
    str = string.gsub (str, " ", "+")
  end
  return str	
end

Convert words in all upper-case to lower-case

str = string.gsub (str, "%f[%a]%u+%f[%A]", string.lower)

Note the use here of the "frontier" regular expression patter %f. Without it it is hard to match on a word boundary, including where the boundary is at the start or end of the string to be matched. Try it on the string "AAA bbb CCC dddEEE FFFhhh JJJ". For more details read about the FrontierPattern.

Match Email addresses

email="alex@it-rfc.de"
if (email:match("[A-Za-z0-9%.%%%+%-]+@[A-Za-z0-9%.%%%+%-]+%.%w%w%w?%w?")) then
print(email .. " is a valid email address")
end

Ruby- and python-like string formatting with % operator

Both ruby and python have a short form for string formatting, using the % operator.

The following snippet adds a similar use of the mod operator to lua :

getmetatable("").__mod = function(a, b)
        if not b then
                return a
        elseif type(b) == "table" then
                return string.format(a, unpack(b))
        else
                return string.format(a, b)
        end
end

Example usage:

print( "%5.2f" % math.pi )

print( "%-10.10s %04d" % { "test", 123 } )

You might like or dislike this notation, choose for yourself.