Expressive Words solution and explanation

def expressive_words(s, words):
    def get_char_groups(word):
        groups = []
        last = word[0]
        count = 0
        for char in word:
            if char == last:
                count += 1
            else:
                groups.append((last, count))
                last = char
                count = 1
        groups.append((last, count))
        return groups

    def is_stretchy(s_group, word_group):
        if len(s_group) != len(word_group):
            return False
        for (s_char, s_count), (w_char, w_count) in zip(s_group, word_group):
            if s_char != w_char:
                return False
            if s_count < w_count:
                return False
            if s_count < 3 and s_count != w_count:
                return False
        return True

    s_group = get_char_groups(s)
    result = 0
    for word in words:
        word_group = get_char_groups(word)
        if is_stretchy(s_group, word_group):
            result += 1
    return result

function expressiveWords(s, words) {
    const getCharGroups = (word) => {
        const groups = [];
        let last = word[0];
        let count = 0;
        for (let char of word) {
            if (char === last) {
                count++;
            } else {
                groups.push([last, count]);
                last = char;
                count = 1;
            }
        }
        groups.push([last, count]);
        return groups;
    };

    const isStretchy = (sGroup, wordGroup) => {
        if (sGroup.length !== wordGroup.length) return false;
        for (let i = 0; i < sGroup.length; i++) {
            const [sChar, sCount] = sGroup[i];
            const [wChar, wCount] = wordGroup[i];
            if (sChar !== wChar) return false;
            if (sCount < wCount) return false;
            if (sCount < 3 && sCount !== wCount) return false;
        }
        return true;
    };

    const sGroup = getCharGroups(s);
    let result = 0;
    for (let word of words) {
        const wordGroup = getCharGroups(word);
        if (isStretchy(sGroup, wordGroup)) {
            result++;
        }
    }
    return result;
}

import java.util.regex.*;

public class Solution {
    public int expressiveWords(String S, String[] words) {
        String pattern = getPattern(S);
        int result = 0;
        for (String word: words) {
            if (isStretchy(word, pattern))
                result++;
        }
        return result;
    }

    private String getPattern(String s) {
        StringBuilder pattern = new StringBuilder();
        int i = 0;
        while (i < s.length()) {
            char c = s.charAt(i);
            int start = i;
            while (i < s.length() && s.charAt(i) == c)
                i++;
            int count = i - start;
            if (count > 2) {
                pattern.append(c).append("{1,").append(count).append("}");
            } else {
                pattern.append(c).append("{").append(count).append("}");
            }
        }
        return pattern.toString();
    }

    private boolean isStretchy(String word, String pattern) {
        return Pattern.matches(pattern, word);
    }
}

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;

public class Solution {
    public int ExpressiveWords(string S, string[] words) {
        var pattern = GetPattern(S);
        int result = 0;
        foreach (var word in words) {
            if (IsStretchy(word, pattern))
                result++;
        }
        return result;
    }

    private string GetPattern(string s) {
        var groupedPattern = new List<string>();
        int i = 0;
        while (i < s.Length) {
            char c = s[i];
            int start = i;
            while (i < s.Length && s[i] == c)
                i++;
            int length = i - start;
            if (length > 2) {
                groupedPattern.Add(c + "{1," + length + "}");
            }
            else {
                groupedPattern.Add(c + "{" + length + "}");
            }
        }
        return string.Join(string.Empty, groupedPattern);
    }

    private bool IsStretchy(string word, string pattern) {
        return Regex.IsMatch(word, "^" + pattern + "$");
    }
}

class Solution {
public:
    int expressiveWords(string s, vector<string>& words) {
        auto check = [](string& s, string& t) -> int {
            int m = s.size(), n = t.size();
            if (n > m) return 0;
            int i = 0, j = 0;
            while (i < m && j < n) {
                if (s[i] != t[j]) return 0;
                int k = i;
                while (k < m && s[k] == s[i]) ++k;
                int c1 = k - i;
                i = k, k = j;
                while (k < n && t[k] == t[j]) ++k;
                int c2 = k - j;
                j = k;
                if (c1 < c2 || (c1 < 3 && c1 != c2)) return 0;
            }
            return i == m && j == n;
        };

        int ans = 0;
        for (string& t : words) ans += check(s, t);
        return ans;
    }
};

func expressiveWords(s string, words []string) (ans int) {
	check := func(s, t string) bool {
		m, n := len(s), len(t)
		if n > m {
			return false
		}
		i, j := 0, 0
		for i < m && j < n {
			if s[i] != t[j] {
				return false
			}
			k := i
			for k < m && s[k] == s[i] {
				k++
			}
			c1 := k - i
			i, k = k, j
			for k < n && t[k] == t[j] {
				k++
			}
			c2 := k - j
			j = k
			if c1 < c2 || (c1 != c2 && c1 < 3) {
				return false
			}
		}
		return i == m && j == n
	}
	for _, t := range words {
		if check(s, t) {
			ans++
		}
	}
	return ans
}