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# Regular Expression
Regular expression (shortened *regex* or *regexp*) is a kind of mathematical [expression](expression.md), very often used in [programming](programming.md), that can be used to define simple patterns in [strings](string.md) of characters (usually text). Regular expressions are typically used for searching patterns (i.e. not just exact matches but rather sequences of characters which follow some rules, e.g. numeric values), substitutions (replacement) of such patterns, describing [syntax](syntax.md) of computer languages, their [parsing](parsing.md) etc. (though they may also be used in more wild ways, e.g. for generating strings). Regular expression is itself a string of symbols which however describes potentially many (even [infinitely](infinite.md) many) other strings thanks to containing special symbols that may stand for repetition, alternative etc. For example `a.*.b` is a regular expression describing a string that starts with letter `a`, which is followed by a sequence of at least one character and then ends with `b` (so e.g. `aab`, `abbbb`, `acaccb` etc.).
Regular expression (shortened *regex* or *regexp*) is a kind of [mathematical](math.md) [expression](expression.md), very often used in [programming](programming.md), that can be used to define simple patterns in [strings](string.md) of characters (usually text). Regular expressions are typically used for searching patterns (i.e. not just exact matches but rather sequences of characters which follow some rules, e.g. numeric values), substitutions (replacement) of such patterns, describing [syntax](syntax.md) of computer languages, their [parsing](parsing.md) etc. (though they may also be used in more wild ways, e.g. for generating strings). Regular expression is itself a string of symbols which however describes potentially many (even [infinitely](infinite.md) many) other strings thanks to containing special symbols that may stand for repetition, alternative etc. For example `a.*.b` is a regular expression describing a string that starts with letter `a`, which is followed by a sequence of at least one character and then ends with `b` (so e.g. `aab`, `abbbb`, `acaccb` etc.).
WATCH OUT: do not confuse regular expressions with Unix [wildcards](wildcard.md) used in file names (e.g. `sourse/*.c` is a wildcard, not a regexp).
WATCH OUT: do not confuse regular expressions with [Unix](unix.md) [wildcards](wildcard.md) used in file names (e.g. `sourse/*.c` is a wildcard, not a regexp).
{ A popular online tool for playing around with regular expressions is https://regexr.com/, though it requires JS and is bloated; if you want to stay with Unix, just grep (possibly with -o to see just the matched string). ~drummyfish }