Regex examples

From Wikipedia: A regular expression (shortened as regex or regexp) is a sequence of characters that specifies a match pattern in text. Basically, you can use regex patterns to efficiently find fine-tuned substring matches in strings.

Each of the examples reads the text in sample.txt and searches for matches that correspond to the regex /^h.\D\S[a-z]{3,5}(...)\w+\d*$/m (although the specific format of this regex differs between all of the languages). Let's break down this regex:

• /***/m is the overall form of a regex pattern with the multiline option enabled
  • This form may not be understood by every programming language, but is the theoretical definition
  • The multiline regex option tells the regex engine to interpret each visual line of text as its own line, rather than the entire string as one "line" of text
• ^ means the start of a line; therefore, a match must start with the letter h
• . means any character, so any character can follow h
• \D means any non-digit, so the third character in a match must not be a number
• \S means any non-whitespace character, so the fourth character in a match must not be whitespace (tabs or \t, spaces, or newlines or \n)
• [a-z]{3,5} means that between 3 to 5 (inclusive) characters in the range a-z (they must all be lowercase) must follow the fourth character
• (...) is what is being "captured" by this regex: the 3 characters following the [a-z]{3,5}
• \w+ means one or more word characters (letters, digits, or underscores) must follow the captured characters in a match
• \d* means zero or more digits must follow the above one or more word characters in a match
• $ means the end of a line

The capture group should be "mei" for both lines in sample.text, but the indices should obviously differ.

The second regex is bob$, which looks for lines that end with "bob". This is used to replace "bob" at the end of a line with "alice".

The third regex is hello[0-9]+world, which just means look anywhere in the string for "hello", then one or more digits in the range 0-9, followed by "world".

The fourth regex is ^he[^A-Z]*\d{2}wo\w{2,}$. Aside from the meanings outlined above:

• [^A-Z]* means zero or more characters that are not in the range A-Z (uppercase letters)
• \d{2} means exactly two digits
• \w{2,} means two or more word characters

The fifth regex is \W+, which means one or more non-word characters. This is used to split a string into substrings that exclude any non-word characters (e.g. punctuation marks and whitespace).

Note: all but the C examples use Perl regex syntax, but the C example uses POSIX ERE (extended regular expressions) syntax. This is also reflected in the GNU and BSD versions of grep -E, respectively. To use Perl regex syntax in C, the libpcre3 library is available.

To compile the C example on Windows (which does not have regex.h as part of its C runtime), install libsystre for Mingw-w64: pacman -S mingw-w64-ucrt-x86_64-libsystre