diff --git a/01_lexer/main.py b/01_lexer/main.py
new file mode 100644
index 0000000..009bac4
--- /dev/null
+++ b/01_lexer/main.py
@@ -0,0 +1,113 @@
+#!/bin/env python3
+
+import argparse
+import datetime
+import ply.lex as lex
+
+tokens = (
+    'LPAREN',
+    'RPAREN',
+    'LSQUARE',
+    'RSQUARE',
+    'LCURLY',
+    'RCURLY',
+    'APOSTROPHE',
+    'AMPERSAND',
+    'COMMA',
+    'DOT',
+    'EQ',
+    'LT',
+    'PLUS',
+    'MINUS',
+    'MULT',
+    'DIV',
+    'STRING',
+    'DATE_LITERAL',
+    'INT_LITERAL',
+    'IDENT',
+    'FUNC_IDENT',
+    'PROC_IDENT',
+)
+
+def t_whitespace(t):
+    r'[ \t\n]+'
+    t.lexer.lineno += t.value.count('\n')
+
+def t_comment(t):
+    r'\(%(.|\n)*%\)'
+    t.lexer.lineno += t.value.count('\n')
+
+t_LPAREN        = r'\('
+t_RPAREN        = r'\)'
+t_LSQUARE       = r'\['
+t_RSQUARE       = r'\]'
+t_LCURLY        = r'\{'
+t_RCURLY        = r'\}'
+t_APOSTROPHE    = r'\''
+t_AMPERSAND     = r'&'
+t_COMMA         = r','
+t_DOT           = r'\.'
+t_EQ            = r'='
+t_LT            = r'<'
+t_PLUS          = r'\+'
+t_MINUS         = r'-'
+t_MULT          = r'\*'
+t_DIV           = r'/'
+
+def t_STRING(t):
+    r'".*?"'
+    t.value = t.value[1:-1]
+    return t
+
+def t_DATE_LITERAL(t):
+    r'\d{4}-\d{2}-\d{2}'
+    t.value = datetime.date.fromisoformat(t.value)
+    return t
+
+def t_INT_LITERAL(t):
+    r'-?\d{1,3}(\'\d{3})*'
+    t.value = int(t.value.replace('\'', ''))
+    return t
+
+def t_IDENT(t):
+    r'[a-z][a-zA-Z0-9_]+'
+    return t
+
+def t_FUNC_IDENT(t):
+    r'[A-Z][a-z0-9_]+'
+    return t
+
+def t_PROC_IDENT(t):
+    r'[A-Z]{2}[A-Z0-9_]*'
+    return t
+
+def t_error(t):
+    print(f'Illegal character \'{t.value[0]}\' at line {t.lexer.lineno}')
+    exit(1)
+
+
+def tokenize_file(file_path: str):
+    lexer = lex.lex()
+    with open(file_path, 'r', encoding='utf-8') as file:
+        lexer.input(file.read())
+
+    tok = lexer.token()
+    while tok:
+        print(tok)
+        tok = lexer.token()
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+    group = parser.add_mutually_exclusive_group(required=True)
+    group.add_argument('--who', action='store_true', help='print out student IDs and NAMEs of authors')
+    group.add_argument('-f', '--file', help='filename to process')
+
+    args = parser.parse_args()
+    
+    if args.who:
+        print('Author')
+        print('  Student ID: 150189237')
+        print('  Name:       Oskari Alaranta')
+    else:
+        tokenize_file(args.file)