Just a toy repo to test stuff with regexp, expose a small script that mimic GNU/grep using the best regexp engine available.

Available sequences are:

• (), group. Used to group expression together, create sub-patterns.
• |, union. Used for choices, match specifically one group out of the available choices.
• *, kleene star. Used for repetition, match the last character or group zero, one or multiple times.
• Σ, sigma. Used as catchall, match any character.
• ε, epsilon. Used as bypass, read nothing.
• \, escape. Used to use the next character as-is.

Available extended sequences are:

• [abc], expand to (a|b|c)
• [1-5], expand to (1|2|3|4|5)
• \s, expand to ( |\n|\r|\t)
• \d, expand to [0-9]
• \w, expand to [a-zA-Z0-9_]

Not available sequences are:

• , match one or multiple times. Can be achieved by prefixing the character to a kleene: aa*.
• ?, match zero or one time. Can be achieved using epsilon: (a|ε)
• {n}, match exactly n times. Can be achieved by concatenate n times the group or character.
• {n,m}, match between n and m times. Can be achieved with n concatenations and m-n epsilon unions.

Example: 0b(0|1)(0|1)* matches python binary numbers.

usage: regexp [-h] [-q] [-x] regexp files [files ...]

positional arguments:
  regexp           Pattern to use
  files            Files to search

optional arguments:
  -h, --help       show this help message and exit
  -q, --quiet      Don't output lines found
  -x, --fullmatch  Match the pattern against a full line