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A modern and consistent Common Lisp string manipulation library

(ql:quickload "str")

also on Ultralisp.

Why ?

  • modernity, simplicity and discoverability:

    • (str:trim s) instead of (string-trim '(#\Space #\Newline #\Backspace #\Tab #\Linefeed #\Page #\Return #\Rubout) s)), or str:concat strings instead of an unusual format construct; one discoverable library instead of many;
  • consistence and composability, where s is always the last argument, which makes it easier to feed pipes and arrows.

  • fixing built-in surprises: (string-downcase nil) => "nil" the string, whereas (str:downcase nil) => nil.

The only dependency is cl-ppcre.

Table of Contents


Install with Quicklisp:

(ql:quickload :str)

Add it in your .asd's project dependencies, and call functions with the str prefix. It is not recommended to :use :str in a package. It's safer to use the str prefix.

Check its version:


To get a newer version, you need to update the Quicklisp dist (think of QL as Debian's apt rather than pip/npm/etc):

(ql:update-dist "quicklisp")

Don't have a full Common Lisp development environment yet ? Get Portacle, a portable and multiplatform development environment shipping Emacs, Quicklisp, SBCL and Git. See also editor support (Vim, Lem, Atom, Eclipse,…).

Global parameters

Some parameters are common to various functions and often used: :ignore-case and :omit-nulls.

Consequently we can also manage them with global parameters:

(let ((str:*ignore-case* t))
  (str:ends-with? "BAR" "foobar"))

is equivalent to

(str:ends-with? "BAR" "foobar" :ignore-case t)


Tweak whitespace

trim (s)

Remove whitespaces at the beginning and end of s.

(trim "  rst  ") ;; => "rst"

Also trim-left and trim-right.

Uses the built-in string-trim where whitespaces are '(#\Space #\Newline #\Backspace #\Tab #\Linefeed #\Page #\Return #\Rubout).

collapse-whitespaces (s)

Ensure there is only one space character between words. Remove newlines.

(collapse-whitespaces "foo  bar

;; "foo bar baz"

To longer strings

join (separator list-of-strings)

Join strings in list list-of-strings with separator (either a string or a char) in between.

(join " " '("foo" "bar" "baz")) ;; => "foo bar baz"
(join #\Space '("foo" "bar" "baz")) ;; => "foo bar baz"

It uses the { iteration format directive. See also this quick reference:

(format nil "~{~a+~}" '(:a :b :c))  ;; the + is the separator
;; => "A+B+C+"

We use the caret directive ("escape upward") to not print the separator in the end:

(format nil "~{~a~^+~}" '(:a :b :c))
;; => "A+B+C"

concat (&rest strings)

Join strings into one.

(concat "f" "o" "o") ;; => "foo"

Simple call of the built-in concatenate.

We actually also have uiop:strcat.

insert (string/char index s)

Insert the given string (or character) at the index index into s and return a new string.

If index is out of bounds, just return s.

(str:insert "l" 2 "helo") ; => "hello"

(str:insert "o" 99 "hell") : => "hell"

repeat (count s)

Make a string of s repeated count times.

(repeat 3 "foo") ;; => "foofoofoo"

add-prefix, add-suffix (items s)

Respectively prepend or append s to the front of each item.

pad (len s &key (pad-side :right) (pad-char #\Space)), pad-left, pad-right, pad-center (new in 0.16, 2019/12)

Fill s with characters until it is of the given length. By default, add spaces on the right:

(str:pad 10 "foo")
"foo       "
  • pad-side: one of :right (the default), :left or :center. See *pad-side*.
  • pad-char: the padding character (or string of one character). Defaults to a space. See *pad-char*.
(str:pad 10 "foo" :pad-side :center :pad-char "+")

If the given length is smaller than the length o s, return s.

Filling with spaces can easily be done with format:

(format nil "~va" len s) ;; => "foo       "
(format nil "~v@a" 10 "foo") ;; => "       foo" (with @)

To shorter strings

substring (start end s)

Return the substring of s from start to end.

It uses subseq with differences:

  • argument order, s at the end
  • start and end can be lower than 0 or bigger than the length of s.
  • for convenience end can be nil or t to denote the end of the string.


  (is "abcd" (substring 0 t "abcd") "t denotes the end of the string")
  (is "abcd" (substring 0 nil "abcd") "nil too")
  (is "abcd" (substring 0 100 "abcd") "end can be too large")
  (is "abc" (substring 0 -1 "abcd") "end can be negative. Counts from the end.")
  (is "" (substring 0 -100 "abcd") "end can be negative and too low")
  (is "" (substring 100 1 "abcd") "start can be too big")
  (is "abcd" (substring -100 4 "abcd") "start can also be too low")
  (is "" (substring 2 1 "abcd") "start is bigger than end")

s-first (s)

Return the first letter of s.


  (s-first "foobar") ;; => "f"
  (s-first "") ;; => ""

s-last (s)

Return the last letter of s.

s-rest (s)

Return the rest substring of s.


  (s-rest "foobar") ;; => "oobar"
  (s-rest "") ;; => ""

s-nth (n s)

Return the nth letter of s.


  (s-nth 3 "foobar") ;; => "b"
  (s-nth 3 "") ;; => ""

You could also use

(elt "test" 1)
;; => #\e
(string (elt "test" 1))
;; => "e"

shorten (len s &key ellipsis)

If s is longer than len, truncate it and add an ellipsis at the end (... by default). s is cut down to len minus the length of the ellipsis (3 by default).

Optionally, give an :ellipsis keyword argument. Also set it globally with *ellipsis*.

(shorten 8 "hello world")
;; => "hello..."
(shorten 3 "hello world")
;; => "..."
(shorten 8 "hello world" :ellipsis "-")
;; => "hello w-"
(let ((*ellipsis* "-"))
  (shorten 8 "hello world"))
;; => "hello w-"

To and from lists

words (s)

Return list of words, which were delimited by whitespace.

unwords (strings)

Join the list of strings with a whitespace.

lines (s &key omit-nulls)

Split string by newline character and return list of lines.

A terminal newline character does not result in an extra empty string (new in v0.14, october 2019).

unlines (strings)

Join the list of strings with a newline character.

split (separator s &key omit-nulls limit start end)

Split into subtrings (unlike cl-ppcre, without a regexp). If omit-nulls is non-nil, zero-length substrings are omitted.

(split "+" "foo++bar") ;; => ("foo" "" "bar")
(split #\+ "foo++bar") ;; => ("foo" "" "bar")
(split "+" "foo++bar" :omit-nulls t) ;; => ("foo" "bar")

cl-ppcre has an inconsistency such that when the separator appears at the end, it doesn't return a trailing empty string. But we do since v0.14 (october, 2019).

rsplit (separator s &key limit)

Similar to split, but split from the end. In particular, this will be different from split when a :limit is provided, but in more obscure cases it can be different when there are multiple different ways to split the string.

(rsplit "/" "/var/log/mail.log" :limit 2) ;; => ("/var/log" "mail.log")
(cl-ppcre:split " " "a b c ")
("a" "b" "c")

(str:split " " "a b c ")
("a" "b" "c" "")


Because it is a common pattern and it can be clearer than an option coming after many parenthesis.

To and from files

from-file (filename)

Read the file and return its content as a string.

Example: (str:from-file "path/to/file.txt").

:external-format: if nil, the system default. Can be bound to :utf-8.

But you might just call uiop's uiop:read-file-string directly.

There is also uiop:read-file-lines.

to-file (filename s)

Write the string s to the file filename. If the file does not exist, create it, if it already exists, replace it.


  • :if-does-not-exist: :create (default), :error
  • :if-exists: :supersede (default), :append, :overwrite, :rename, :error,...

Returns the string written to file.


empty?, emptyp (s)

True if s is nil or the empty string:

  (empty? nil) ;; => T
  (empty? "")  ;; => T
  (empty? " ") ;; => NIL

See also str:non-empty-string-p, which adds a stringp check.

blank?, blankp (s)

True if s is empty or only contains whitespaces.

(blankp "") ;; => T
(blankp " ") ;; => T
(emptyp " ") ;; => NIL

See also str:non-blank-string-p.

starts-with?, starts-with-p (start s &key ignore-case)

True if s starts with the substring start, nil otherwise. Ignore case by default.

(starts-with? "foo" "foobar") ;; => T
(starts-with? "FOO" "foobar") ;; => NIL
(starts-with? "FOO" "foobar" :ignore-case t) ;; => T

Calls string= or string-equal depending on the case, with their :start and :end delimiters.

ends-with?, ends-with-p (end s &key ignore-case)

True if s ends with the substring end. Ignore case by default.

(ends-with? "bar" "foobar") ;; => T

contains?, containsp (substring s &key (ignore-case nil))

Return true if s contains substring, nil otherwise. Ignore the case with :ignore-case t (don't ignore by default).

Based on a simple call to the built-in search (which returns the position of the substring).

s-member (list s &key (ignore-case *ignore-case*) (test #'string=))

Return T if s' is a member of list'. Do not ignore case by default.

NOTE: s-member's arguments' order is the reverse of CL's member.

If :ignore-case or *ignore-case* are not nil, ignore case (using string-equal instead of string=).

Unlike CL's member, s-member returns T or NIL, instead of the tail of LIST whose first element satisfies the test.

prefix?, prefixp and suffix?, suffixp (items s)

Return s if it is a common prefix (or suffix) between items.

See also uiop:string-prefix-p prefix s, which returns t if prefix is a prefix of s,

and uiop:string-enclosed-p prefix s suffix, which returns t if s begins with prefix and ends with suffix.


Functions to change case: camel-case, snake-case,... (new in 0.15, 2019/11)

We use cl-change-case (go thank him and star the repo!).

The available functions are:

:no-case (s &key replacement)
:camel-case (s &key merge-numbers)

More documentation and examples are there.

downcase, upcase, capitalize (s) fixing a built-in suprise. (new in 0.11)

The functions str:downcase, str:upcase and str:capitalize return a new string. They call the built-in string-downcase, string-upcase and string-capitalize respectively, but they fix something surprising. When the argument is nil, the built-ins return "nil" or "NIL" or "Nil", a string. Indeed, they work on anything:

(string-downcase nil) ;; => "nil" the string !
(str:downcase nil) ;; nil

(string-downcase :FOO) ;; => "foo"

downcasep, upcasep (s)

These functions return t if the given string contains at least one letter and all its letters are lowercase or uppercase, respectively.

(is (downcasep " a+,. ") t "downcasep with one letter and punctuation is true.")
(is (downcasep " +,. ") nil "downcasep with only punctuation or spaces is false")

alphap, lettersp (s)

alphap returns t if s contains at least one character and all characters are alpha (as in "^[a-zA-Z]+$").

lettersp works for unicode letters too.

(is (alphap "abcdeé") nil "alphap is nil with accents")
(is (lettersp "éß") t "lettersp is t with accents and ß")

alphanump, lettersnump (s)

alphanump returns t if s contains at least one character and all characters are alphanumeric (as in ^[a-zA-Z0-9]+$).

lettersnump also works on unicode letters (as in ^[\\p{L}a-zA-Z0-9]+$).

digitp (s)

Returns t if s contains at least one character and all characters are numerical (as for digit-char-p).

has-alpha-p, has-letters-p, has-alphanum-p (s)

Return t if s has at least one alpha, letter, alphanum character (as with alphanumericp).


replace-first (old new s)

Replace the first occurence of old by new in s. Arguments are not regexs.

(replace-first "a" "o" "faa") ;; => "foa"

Uses cl-ppcre:regex-replace but quotes the user input to not treat it as a regex.

replace-all (old new s)

Replace all occurences of old by new in s. Arguments are not regexs.

(replace-all "a" "o" "faa") ;; => "foo"

Uses cl-ppcre:regex-replace-all but quotes the user input to not treat it as a regex.

replace-using (plist s)

Replace all associations given by pairs in a plist and return a new string.

The plist is a list alternating a string to replace (case sensitive) and its replacement.


(replace-using (list "%phone%" "987")
               "call %phone%")
;; "call 987"

remove-punctuation (s &key replacement)

Remove the punctuation characters from s, replace them with replacement (defaults to a space) and strip continuous whitespace.

(str:remove-punctuation "I say: - 'Hello, world?'") ;; => "I say Hello world"

Use str:no-case to remove punctuation and return the string as lower-case.

prefix (list-of-strings) (renamed in 0.9)

(renamed from common-prefix in v0.9)

Find the common prefix between strings.

Example: (str:prefix '(\"foobar\" \"foozz\")) => "foo"

Uses the built-in mismatch, that returns the position at which the strings fail to match.

Return a string or nil when the input is the void list.

suffix (list-of-strings)

Find the common suffix between strings.

count-substring (substring s &key start end)

Counts the non-overlapping occurrences of substring in s. You could also count only the ocurrencies between start and end.


(count-substring "abc" "abcxabcxabc")
;; => 3
(count-substring "abc" "abcxabcxabc" :start 3 :end 7)
;; => 1

s-assoc-value (alist key)

Returns the value of a cons cell in alist with key key, when key is a string. The second return value is the cons cell, if any was matched.

The arguments are in the opposite order of cl:assoc's, but are consistent with alexandria:assoc-value (and str).

(s-assoc-value '(("hello" . 1)) "hello")
;; 1
;; ("hello" . 1)

(alexandria:assoc-value '(("hello" . 1)) "hello")
;; NIL
(alexandria:assoc-value '(("hello" . 1)) "hello" :test #'string=)
;; 1
;; ("hello" . 1)

(assoc "hello" '(("hello" . 1)))
;; NIL
(assoc "hello" '(("hello" . 1)) :test #'string=)
;; ("hello" . 1)
(cdr *)
;; 1



A case-like macro that works with strings (CL case's test function is eql, and that isn't enough for strings).


(str:string-case input
  ("foo" (do something))
  (nil (print "input is nil")
  (otherwise (print "non of the previous forms was caught.")))

You might also like pattern matching. The example below with trivia is very similar:

(trivia:match "hey"
  ("hey" (print "it matched"))
  (otherwise :nothing))

Note that there is also


  • 0.19, October, 2020: added s-member *0.18.1, September, 2020: fix replace-all edge case when the replacement string ends with two backslashes and a single quote.
  • 0.18, June, 2020: added replace-using.
  • 0.17, April 2020:
    • added collapse-whitespaces
    • join and split also accept a char as separator
    • fixed remove-punctuation that did not respect the case. Use no-case for this
    • fixed from-file "odd number of arguments" error.
  • 0.16, November 2019: added pad, pad-[left, right, center].
  • 0.15, October 2019: added functions to change case (based on cl-change-case). added remove-punctuation.
  • 0.14, October, 2019: fixed the cl-ppcre inconsistency in split and lines. A trailing separator now returns a trailing empty string.


(str:split " " "a b c ")
("a" "b" "c")  ;; like cl-ppcre:split


(str:split " " "a b c ")
("a" "b" "c" "")
  • august, 2019: deprecated prune, renamed to shorten.
  • added :limit to split.
  • 0.13 june, 2019
    • added insert
  • 0.12
    • added case predicates (downcasep, alphap, has-x and friends).
  • 0.11 (Quicklisp end of march, 2019, also in Ultralisp)
    • added str:downcase, str:upcase and str:capitalize, that fix the nil argument surprise.
  • 0.10
    • split doesn't fix cl-ppcre's inconsistency anymore (when the separator appears at the end). See issue #18. So (str:split "xx" "fooxxbarxx") doesn't return a trailing "".
    • added s-last
    • s-first and friends return nil when appropriate, not "".
  • 0.9
    • added s-first , s-rest and s-nth
    • added prefix and suffix functions and predicates.
    • added prune.
  • 0.8 added string-case
  • 0.7 added version
  • 0.6 added split-omit-nulls (QL, january 2018)
  • 0.5 added common-prefix
  • 0.4 added from-file and to-file.
  • 0.3 added substring.

Dev and test

Test with prove.

(ql:quickload :str.test) (load "test/test-str.lisp")

See also

Inspired by the famous Emacs Lisp's s.el.