1. grep.js

GNU grep compiled to JavaScript

grep searches input files for lines containing a match to a given pattern list. When it finds a match in a line, it copies the line to standard output (by default), or produces whatever other sort of output you have requested with options. Though grep expects to do the matching on text, it has no limits on input line length other than available memory, and it can match arbitrary characters within a line. If the final byte of an input file is not a newline, grep silently supplies one. Since newline is also a separator for the list of patterns, there is no way to match newline characters in a text.

May be invoked with the following command-line options:

2. Regular Expressions

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions. grep understands three different versions of regular expression syntax: “basic” (BRE), “extended” (ERE) and “perl” (PCRE). In GNU grep, there is no difference in available functionality between the basic and extended syntaxes. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.

2.1. Fundamental Structure

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any meta-character with special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of several repetition operators:

The empty regular expression matches the empty string. Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated expressions.

Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either alternate expression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression. An unmatched ‘)’ matches just itself.

2.2. Character Classes and Bracket Expressions

A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list; if the first character of the list is the caret ^, then it matches any character not in the list. For example, the regular expression [0123456789] matches any single digit.

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive. In the default C locale, the sorting sequence is the native character order; for example, [a-d] is equivalent to [abcd].

Finally, certain named classes of characters are predefined within bracket expressions, as follows.


Most meta-characters lose their special meaning inside bracket expressions.

2.3. The Backslash Character and Special Expressions

The \ character, when followed by certain ordinary characters, takes a special meaning:



For example, \brat\b matches the separate word rat, \Brat\B matches crate but not furry rat.

2.4. Anchoring

The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line. They are termed anchors, since they force the match to be “anchored” to beginning or end of a line, respectively.

2.5. Back-references and Subexpressions

The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression. For example, (a)\1 matches aa. When used with alternation, if the group does not participate in the match then the back-reference makes the whole match fail. For example, a(.)|b\1 will not match ba. When multiple regular expressions are given with -e back-references are local to each expression.

2.6. Basic vs Extended Regular Expressions


Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {

GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification. For example, the command grep -E '{1' searches for the two-character string {1instead of reporting a syntax error in the regular expression. POSIX allows this behavior as an extension, but portable scripts should avoid it.

Default Output

GIST | grep sends its results to the screen by default

Basic Usage

GIST | We can use grep to search for every line that contains the word 'Adams'

GIST | If we would want grep to ignore the "case" of our search parameter and search for both upper- and lower-case variations, we can specify the -i or --ignore-case option

GIST | If we want to find all lines that do not contain a specified pattern, we can use the -v or --invert-match option.

GIST | It is often useful to know the line number that the matches occur on. This can be accomplished by using the -n or --line-number option.

Regular Expressions

GIST | Anchors are special characters that specify where in the line a match must occur to be valid. For instance, this string example will only mach "J" if it occurs at the very beginning of a line

GIST | Similarly, the $ anchor can be used after a string to indicate that the match will only be valid if it occurs at the very end of a line.

GIST | The period character . is used in regular expressions to mean that any single character can exist at the specified location.

GIST | By placing a group of characters within brackets [ and ], we can specify that the character at that position can be any one character found within the bracket group. This means that if we wanted to find the lines that contain "01" or "09", we could specify those variations succinctly by using the following pattern:

GIST | We can have the pattern match anything except the characters within a bracket by beginning the list of characters within the brackets with a ^ character.

GIST | One of the most commonly used meta-characters is the *, which means "repeat the previous character or expression zero or more times". If we wanted to find each line that contained an opening and closing parenthesis, with only letters and single spaces in between, we could use the following expression:

GIST | We can escape characters by using the backslash character (\) before the character that would normally have a special meaning

GIST | Similar to how bracket expressions can specify different possible choices for single character matches, alternation allows you to specify alternative matches for strings or expression sets. To indicate alternation, we use the pipe character |

GIST | To match a character zero or one times, you can use the ? character. This makes character or character set that came before optional, in essence

GIST | The + character matches an expression one or more times. This is almost like the * meta-character, but with the + character, the expression must match at least once. The following expression matches the string "free" plus one or more characters that are not whitespace.

GIST | If we want to match any words that have between 5 and 7 characters, we can use the following expression:

Further Reading