IRM

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Standard

IRM (Mode 4) is a DECSET/DECRST mode that controls whether newly typed characters insert at the cursor position (shifting existing characters to the right) or replace the character under the cursor.

Syntax

CSI ? 4 h    DECSET — Enable insert mode
CSI ? 4 l    DECRST — Disable insert mode (replace mode)

Formal syntax

DECSET-mode4 = 0x1b, "[", "?", "4", "h" ;
DECRST-mode4 = 0x1b, "[", "?", "4", "l" ;

Description

Per ECMA-48 §7.2.10, IRM (Insertion Replacement Mode) controls the effect of a received character on the active presentation position.

Set (insert mode) — Characters received at the cursor position are inserted. Existing characters from the cursor to the right margin shift right by one position. The character at the right margin is lost (it scrolls off the edge). The cursor advances one position to the right.
Reset (replace mode) — Characters received at the cursor position overwrite the character currently at that position. The cursor advances one position to the right. This is the default.

note

IRM is defined as an ANSI standard mode in ECMA-48 and is technically set with CSI 4 h / CSI 4 l (without the ? prefix). However, many terminal emulators accept the private mode form CSI ? 4 h / CSI ? 4 l as well. The ANSI form is more correct per the standard.

Interaction with other features

In insert mode, the ICH (Insert Character) sequence CSI Pn @ inserts blank characters, which is distinct from IRM where printable characters are inserted.
Insert mode does not affect characters written via escape sequences; only printable characters received at the cursor position are subject to insertion behavior.

Examples

printf '\e[4h'       # Enable insert mode (ANSI form)
printf '\e[4l'       # Disable insert mode / enable replace mode (ANSI form)
printf '\e[?4h'      # Enable insert mode (private mode form)
printf '\e[?4l'      # Disable insert mode (private mode form)

Specifications

Specification	Section
ECMA-48	—

Terminal support

Terminal	Support	Version	Notes
Terminal Emulators
Alacritty	✓	Yes
Bobcat	?	?	Not explicitly configured in Bobcat. TerminalCtrl may support this, but cannot confirm from Bobcat's code.
contour	✓	v0.3.0.198	Implemented as AnsiMode::Insert at primitives.h:606, handled in Terminal.cpp:2251
foot	✓	Yes
Ghostty	✓	v1.0.0
iTerm2	✓	v20260216-nightly
Kitty	✓	Yes
Konsole	✓	Yes	MODE_Insert implemented in Screen class
mintty	✓	Yes	Implemented at src/termout.c:2636
mlterm	✓	rel-0_2_1	Present since initial public release
PuTTY	✓	0.45
Rio	✓	v0.1.13
rxvt-unicode	✓	1.2	Insert/Replace mode via scr_insert_mode
st	✓	0.1
terminology	✓	v0.1.0	Uses termstate.insert flag
VT100	✓	Yes
VTE	✓	Yes
WezTerm	✓	Yes
Windows Terminal	✓	Yes	IRM_InsertReplaceMode at adaptDispatch.cpp:1762-1764
xterm	✓	Yes	ANSI insert/replace mode
xterm.js	✓	5.0.0
Multiplexers
cy	✗	No	Mode tracked but logged as not implemented
GNU Screen	✓	v.4.2.0	Present since at least v.4.0.2 (2005)
tmux	✓	0.8
tuios	✗	No
Zellij	✓	v0.12.0	Handled in grid.rs:3016-3120 via CSI 4 h/l

Syntax​

Description​

Interaction with other features​

Examples​

Specifications​

Terminal support​

See also​