How to match a pattern and String without using regular expressions in Python

Hello learners, here we are going to learn how to match a pattern and string without regular expressions (regex) in Python. Some may get wrong in understanding the title. Let us see this example to make it simple.

Example-1:

String : BigBunnyBig
Pattern: FGF
Output: Pattern Found
f - BIG
G - Bunny

Example-2:

String : BigBunnyBig
Pattern: FG
Output: Pattern not matched

Now it is clear about pattern matching. Let’s get into the implementation.

The logic is that separate the string into words by the conditions like uppercase or space separated words etc. you may get different ideas to separate. that’s up to your choice.

Implementation

def pattern_matching(str,pattern,str_len,pat_len):
    #if length of pattern is more than length of string,
    #then the pattern doesn't matches with string
    if(str_len<pat_len):
        return False
    #if length of string and pattern is 0 then we set it to match
    if(str_len==0 and pat_len==0):
        return True
    #if one is empty and the other is entered then it is mismatched
    if (str_len == 0 or pat_len == 0):
        return False
    #check if string is space separated one or not
    # if yes then split() function can be used. it just separate the string with the space
    #if no check the else condition
    if(str.find(' ')!=-1):
        str_list=str.split()
    else:
        #check for every characters and check for uppercase letters and add it to the list str_list
        splitted_words = [[str[0]]]
        for i in str[1:]:
            if i.isupper() and splitted_words[-1][-1].islower():
                splitted_words.append(list(i))
            else:
                splitted_words[-1].append(i)
        str_list=[''.join(word) for word in splitted_words]
    #separate the pattern with no repetition for easy logic purpose
    #those are stored in pat_list
    pat_list = []
    for i in pattern:
            if(i not in pat_list):
                pat_list.append(i)
    #like pattern storing the separated string with no repetition
    # store that with in the dictionary with pattern characters as corresponding keys
    j=0
    dict = {}
    for i in pat_list:
        while(j<str_len):
            dict[i]=str_list[j]
            j+=1
            break
    #check for matching conditions
    #if characters of pattern matches with keys in the dictionary
    #then store that values in separate list output
    output=[]
    for i in pattern:
        if(i in dict.keys()):
            output.append(dict[i])
    #if output lined with the pattern matches with string
    #then it matches else not
    if(output==str_list):
        print(dict)
        return True
    else:
        return False
# Example string and pattern
str="CodeSpeedyCode"
pattern="MCM"
#check for matching conditions string and patterns
if(pattern_matching(str,pattern,len(str),len(pattern))):
    #True conditions
    print("Pattern Matching")
else:
    #false conditons
    print("There is no matching found")

Output:

{'M': 'Code', 'C': 'Speedy'}
Pattern Matching

Let’s have this

str="CodeSpeedy"
pattern="MCM"

Output:

There is no matching found

Testcase3:

str="Code Speedy"
pattern="MC"

Output:

{'G': 'Code', 'F': 'Speedy'}
Pattern Matching

Also read: re.search vs re.match in Python